This reverts the changes to Load from commits
  - 67348e40
  - cf334a95

We need a better way for a specific loader to request that it will handle
all files that Load has found.
Refs #10061

In the case of LoadEventPreNexus is is "EventFilename". It seems that for
everything else is is "Filename" because the variable that holds this is
empty by the time the algorithm comes to run (though the loader is
selected correctly).
I've factored out the method that sets the variable and called it again
from within a method that exec() calls.

Avoids loading everything then summing in separate loop, instead loads then sums in same
loop

This reverts commit 4fe9cd9d, and fixes
tests.

This reverts commit a9c8a59a.

Allow filenames specified with the format <<short><operator>>...<short>
where:

<short> = <dir><inst><underscore><runs><ext>
<operator> = "+" or ","
<runs> = lists and/or ranges of (possibly added) runs e.g. "12,13+14,15:20".

See header file of MultipleFileProperty for more info on syntax.

This enables workspaces loaded through the algorithm to have a history that
can be used to reproduce the workspace properly.

MWRunFiles widget of LoadDialog now remembers the exact text a user typed -
in this way any user who types "INST0-50", clicks "Run" and reopens the
dialog is not met by a huge string containing a list of all 51 fully
resolved files, but rather their original input.

Changed the vector<vector<string>> templated versions of the toString and
toValue functions to accept customised delimiters.  This enables us to add
an option for the user to turn off multifile parsing if they are crazy
enough to want to load files with a plus or a comma in their path.

Changed the inner workings of Load for multi files.  No longer calling sub
algorithms with setChild or alwaysStoreInADS.  All workspaces are
essentially created outside the ADS and managed by the algorithm, right up
until the OutputWorkspaces are set.

Tests added where appropriate, but the majority of parsing cases are already
covered by MultiFileNameParser.

MultiFileProperty changed to store a vector of vectors of file names. File
names that share a vector are to be added. Old functionality can be produced
by calling "flattenFileNames".  See source code for more info.

MultiFileProperty was used in one place previously (MDMergeFiles), this has
been modified to relect these changes.

Load now implemented in terms of MultiFileProperty, though its interface is
unchanged.  "Filename" input is now parsed as a multi file, and if that
fails old functionality is attempted.  Error reports will output the result
of both attempts.

Some additional template specialisations / "toString" overloads
surrounding the Property class heirachy were necessary to properly support
std::vector<std::vector<std::string> >.

Refs #3724: Pulled wiki description text from the wiki site and added in algorithms' header file with special tags. The auto_wiki.py script can retrieve the text to generate the wiki page fully.

1) I don't think anything should be done here as fast algorithms may not be possible to cancel anyway.
2) Made Algorithm::cancel() method virtual and overridden it in Load to call cancel() on the concrete loader. A test file can be sans2d00005512.nxs.
3) MantidPlot makes sure that the AlgorithmDockWidget is visible when an algorithm is running. fixes #1733

Fixes #2839. Make the error message from the Load algorithm much clearer if it can't find an appropriate loader.

Refs #2572: Added an initDocs() for all algorithms that sets the summary string to display in the dialog, as well as the wiki summary for possible later auto-wiki-fication of documentation.

Correction to the Load algorithm so that it will pick up any output workspace on a sub algorithm, not just ones whose name starts with OutputWorkspace. Re #2169

Modified LoadEventPreNeXus so that it is compatible with the Load algorithm, Refs #2168. Also made a couple of adjustments to the layout of the MWRunFiles widget to make it better at resizing with the dialogs it is attached to. Re #2329

Implemented a custom dialog for the Load algorithm. It now dynamically generates its properties from those that the concrete loaer has, Refs #2329. Also fixed a problem with loading grouped nexus files through the Load algorithm, Re #2362

This should be everything I need for the Framework to make using the Load algorithm much simpler (including from C++). Re #2329

Some minor changes to neaten up the IDataFileChecker children and to move the static contants that were not attached to anything in particular. Re #2329

Update to the Load algorithm to allow running solely to discover the concrete loader type. Also updated workspace property to respect the optional flag in other cirumstances. Re #2329