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.
Merge pull request #188 from broadinstitute/mc_add_field_conversion_api_175

Variant: separate api for type conversion fields