Inferring paired/unpaired t-test from formula

[EitanHemed]

Issue
When fitting a two-samples t-test using the formula interface, it is required to also provide the paired kwarg, although it should be inferred from the formula.

How to reproduce

import robusta as rst
m = rst.groupwise.groupwise_models.T2SamplesModel(data=rst.load_dataset('sleep'), formula='extra~group|ID')
m.fit()

Output
NotImplementedError: Conversion 'py2rpy' not defined for objects of type '<class 'NoneType'>'

Expected Output