Allow to pass a python dictionary as schema

[jordeu]

Let pass a dictionary following the same layout that we have in the schema file.

Simple example:

schema = { 'POSITION': { 'PARSER': 'int(x)', 'VALIDATOR': 'x > 0' }
data = itab.open('mutations.itab.tsv', schema=schema)

It will also be possible to pass a function instead of a string as parser and validator. This will allow to create more complex custom parsers and validators.

Advance example:

def example_parser(x, row):
    return int(x)

def example_validator(x, row):
    return x > 0

schema = { 'POSITION': { 'PARSER': example_parser, 'VALIDATOR':  example_validator }
data = itab.open('mutations.itab.tsv', schema=schema)