Variant: refactor of the shared field api

* shared fields api now behaves like the individual fields api (creates an object but doesn't make any copies)
* SharedField class is iterable and can hold multiple values
* special cased boolean_shared_field to return a single bool (since * vector<bool> is impossible by spec)
* current API (as well as the individual field API) relies on missing() checks before using the values. Dereferencing a missing shared field will segafult -- see #166
* add check for dereferencing an empty field (throws out of range exception instead of segfault)

fixes #151
With regards to the naming scheme: now that we no longer have the Variant* prefix on these classes, I think we may need to consider creating "variant" and "sam" directories in the gamgee codebase to make it clear which classes are variant-related and which are sam-related. The alternative would be to restore the variant prefix at the cost of awkwardly long names (eg., VariantIndividualField).