Stop going through tumor_build to get reference_sequence_build

This helps make the command compatible for somatic-variation as well as
somatic-validation builds.
Added control/experimental_subject to som-var model

This is for api-compatibility with somatic-validation models and they
have it for APConfig compatiblity.
Removed code duplication with delegation
Use build-type agnostic accessors

This command should now accept either a somatic-validation or a
somatic-validation build as the source_build.