Double declaration of Variant class and it const values

[samanvp]

In PR #322 we moved a couple of const values (namely: MISSING_FIELD_VALUE, PASS_FILTER, END_INFO_KEY, GENOTYPE_FORMAT_KEY, PHASESET_FORMAT_KEY, DEFAULT_PHASESET_VALUE, MISSING_GENOTYPE_VALUE) and classes (MalformedVcfRecord, Variant, VariantCall) from vcfio.py to vcf_parser.py.

The reason was that we wanted to avoid circular imports which made it impossible to run the code because vcfio.py imported vcfio_parser.py and vice versa.

However, since these const values and classes are accessed via vcfio.py by the rest of the code, we redeclared them in vcfio.py to avoid changing the current settings.