Caf bugfix

The Caf interpreter test was not actually testing the new
Number=R caf field.  Updated the tests to actually test this,which
broke.  Fixed how we were calling info_for_allele to return the
expected value.
The existence of both `create_vcf_header` and `new_vcf_header` is a little confusing.  If we get a third VCF version this will get out of hand quickly!