Suggestion: Add process_alt() to __init__.py and documentation

[luederm]

I often use vcfpy to create VCF files from non-vcf data sources, such as a spreadsheet. One difficulty with doing this has been creating AltRecords for alternative alleles. Looking through the code I found the process_alt function, which I find very helpful for this use-case. I think it would help other people looking to create VCFs without a starting VCF if this was in the documentation.