Support 'local allele' encodings

[cmdcolin]

the concept of local alleles was addd in vcf 4.5

the VCF 4.5 spec suggests:

"It is recommended that VCF libraries provide an API in which local allele encoding can be abstracted away
from the API consumer and values accessed through their corresponding non-local key"

https://github.com/samtools/hts-specs/blob/master/VCFv4.5.pdf

this is similar and probably replaces #11