Add validation rules to MutationHandler

[flaneuse]

Refactor MutationHandler to add validation:

1. As before, create a copy of the handler to enable easy testing between the new version of the handler and the existing one
2. Refactor the structure of the parameters to be a BioThings-style kwargs format
3. Add validation rules for the parameters:

• name: str

@newgene: I would also suggest moving out all this function and the other mutation ones in lineage.py into a separate handler file (MutationHandler, LineageMutationsHandler, MutationDetailsHandler, MutationsByLineage)