Use string (instead of int from PyVCF) as the "Number" fields in header

[arostamianfar]

We currently use PyVCF integers [1] for the Number field in VcfHeader. This has caused a lot of confusion in our code, so we should fix it to use a string instead (i.e. avoid the conversion from "A" to -1, etc). Nucleus also uses a string for this field, so it would make the transition smoother.

[1] PyVCF has some arbitrary semantics for special number values:

# Conversion between value in file and Python value
field_counts = {
    '.': None,  # Unknown number of values
    'A': -1,  # Equal to the number of alternate alleles in a given record
    'G': -2,  # Equal to the number of genotypes in a given record
    'R': -3,  # Equal to the number of alleles including reference in a given record
}