reduce DSK memory usage in high number of partitions/cores scenarios:  optimize the number of cached items per partition per core

inside a thread group

level is 0 (means no compression), and the compression level can be set
(from 0 to 9) with the '-out-compress N' option

In case of compilation issues, one can deactivate this by using cmake
-DGATB_CORE_EXCLUDE_HDF5_ZLIB=1 ..

now, if you give a h5 file (instead of reads) as input for a tool which is going to create a Graph, assuming this h5 file contains at the least solid kmers (it may also contain the whole graph), then GATB will finish constructing the graph (according to default/passed parameters). E.g. Minia can take as input a DSK output.

split build_visitor of Graph.cpp into two components. passes all tests. also added setProperty in StorageHDF5

several instances