logic overhaul; abstract inventory creation
logic overhaul; abstract inventory creation

each function was creating its own inventory; this is not necessary
also, if a user wants only to build a single document, it is not necessary to
scan the entire source and output directory (small, but perceptible speedup to
the end user)
better checking of the various different possible argument types, since an
argument can be a filename, a status_class or a stem

tLDP/python-tldp,tLDP/python-tldp,tLDP/python-tldp