ut-pipelining memory: Add test for assignment fairness.
doc/pipelining.dox: Document memory fractions semantics.
Doxygen: Disambiguate \file names.

Source files in tpie/pipelining/ clash with `exception.h`, `stdio.h`,
`file_stream.h` and others which made Doxygen complain.
Brief documentation of pipelining/graph.h classes.