Improvements to handling files and RDF graphs.

* [FILE_EXT,SC] Removed absolute_file_directory/3 which was not used.
* [FILE_EXT,NEW] Added absolute_file_name_number/4 for numbered file names. Thus uses spec_atomic_concat/3 for concatenating an atom to a nested spec.
* [RDF_GRAPH,EXT] rdf_graph_merge/2 now takes a list of both graphs and files containing graphs.
* [RDF_SERIAL,NEW] Added file[s]_or_graph[s]/2 allowing a method to take mixed lists of graphs and files.
* [RDF_SERIAL,EXT] rdf_load2/2 returns the graph/1 and format/1 options (if they were not set).