Improving --gencode_primary and --flag_gencode_primary

[likhitha-surapaneni]

• Added incompatible flags
• Added warnings for unsupported assemblies
• Style changes in description of header for GENCODE_PRIMARY field
• Consistency with public-plugins/pull/812

Testing

vep -i input.vcf --gencode_primary --refseq   # Error 
vep -i input.vcf --gencode_primary --gencode_basic # Error
vep -i input.vcf --gencode_primary --merged # No error

vep -i input.vcf --gencode_primary --assembly GRCh37 # Warning 
vep -i input.vcf --gencode_primary  --flag_gencode_primary --species mus_musculus --assembly GRCm39 # Warning