✨ track ALT and REF alleles in locations

[bunop]

Is your feature request related to a problem? Please describe.
Tracking ALT and REF alleles for each location is required to convert data into VCF format

Describe the solution you'd like
ALT and REF alleles should be determine from genome alignments and then tracked for each Location object

Describe alternatives you've considered
The sheep genome consortium tracks genotypes with REF/ALT convention, data can be imported from here. However, all the SNPs not managed by them (ie Affymetrix) cannot have these attributes

Additional context
Please see comments in #18 for import_consortium.py

• Add ALT and REF class attributes to src.features.smarterdb.Location
• Track latest data from Sheep genome consortium