Refs #1419 - Changes to finally support multi file loading.

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> >.