Informal definition of API via unit tests
VariantCallSupport model
vcs implementation
minor code cleanup
now get_vcs_by_* returns None is no vcs has been found
added delete_vcs
added a simple example
test/genotype moved common test stuff to common.py
Added subindexing to GenotypeDataSample.resolve_to_data()
bug fixes
added __cleanup__
changed PedWriter: now uses vcs, updated relevant tests.
Merge branch 'vcs' into develop.

Adds VariantCallSupport and uodates PedWriter to use it. VCFWriter has not yet been upgraded, so it is currently broken.
bug fix