model.py: add_field function added in Call class, for adding or updating...

I think it could be a good improvement to allow the PyVCF module to be able to add or update information fields in Call objects, that is, in the FORMAT fields of the .vcf files. This feature could make possible to perform other interesting functions like merging of .vcf files coming from different variant callers, etc. I will be glad to include some of these functionalities in future releases.

Changes performed:
model.py: add_field function added in Call class, for adding or updating a field of a call object, that is, it allow to modify calls