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

The converted text is stored in separate .rst files in the docs directory.
Refs #9562

ITableWorkspace has been added to the list of output workspace types load is expecting to output.

Mainly removed Poco includes and used forward declarations instead.
Unsuprisingly this has affected some algorithms.
Refs #8784

* Remove temp workspace at end of summing.
* Ensure output of load is in the workspace it's supposed to be.

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.

Otherwise, when loading sqw, it complains about unknown extension when logging is done in debug mode

Refs #7852

Re #7253

re #6485

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

Ensure that input into a MultipleFileProperty properly survies a "round
trip", i.e. what you put in is what you get out, and that using the output
again, as input into another MultipleFileProperty gives the same results.

Proper functionality in a large number of use cases is assured by vastly
improved coverage of unit tests, which was lacking previously.

Simplify parts of the MultipleFileProperty class, including the filename
generation functors I introduced that weren't doing anybody any favours.