SamIterator: remove unnecessary move construction
VariantIterator: remove unnecessary move construction
Merge pull request #193 from broadinstitute/mc_refactor_fetch_next_record_192

Remove unnecessary move constructions from our iterators
Variant: add index based field getter API

individual and shared fields can be more efficiently accessed via
indices looked up from the header once instead of doing the hash lookups
every time you access a field (during traversal). This commit adds that
functionality.

* add field index API to the VariantHeader
* refactor code duplication in INDIVIDUAL and INFO field API
* avoid double index lookup in the unforgiving field getter API
* add tests for missing indices in the header

ps: tests for the by_index API are written in terms of the field getter
API that accepts conversion. Exceptions are on the tests commented out
that require #187 to be implemented. Hence why a few are not covered.

fixes #162
Address reviewer's comments