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 #3666

refs #3666

refs #3666

Put the construction of a descriptive string into conventional
cell class.
refs #3666

To try to speed up MergeMDFiles, however my second try is 1172 seconds which is slightly slower than before.

refs #3666

They used to show up black due to logic error.

and fix windows build I hope

the other build systems

Because that is more readable

Because that is more readable

Because that is more readable

I think. Later I will replace c-style casts with c++ static_cast<coord_t>

Also added number indexed and average error output
properties fof SelectCellOfType algorithm.
refs #3666

To reduce warnings when coord_t = float. Also, fix a warning #4473.