Add Variant::alignment_stop() accessor
- add tests including GVCF
Merge pull request #185 from broadinstitute/jt_alignment_stop

Add Variant::alignment_stop() accessor
Variant: separate api for type conversion fields

As described in issue #175 we decided to provide two different APIs for
individual and shared field access. One does implicit conversions and
the other one throws exceptions if the type requested is not the same
the field was encoded in.

fixes #175.