VIVO-906 Create the TripleSource hierarchy.

SingleSourceDataStructuresProvider becomes TripleSource
DataStructuresProvider becomes CombinedTripleSource
VIVO-906 Assemble application at runtime.

Create an object to represent VitroHomeDirectory
Instantiate TripleSources, FileStorage, etc, based on the config file.
VIVO-906 Explicitly configure the SearchIndexExcluders and DocumentModifiers.