Graph refuters, support for dagitty, and creating your own estimators

• Graph refuter with conditional independence tests to check whether data conforms to the assumed causal graph

• Better docs for estimators by adding the method-specific parameters directly in its own init method

• Support use of custom external estimators

• Consistent calls for init_params for dowhy and econml estimators

• Add support for Dagitty graphs

• Bug fixes for GLM model, causal model with no confounders, and hotel case-study notebook

Thank you @EgorKraevTransferwise, @ae-foster, and @anusha0409 for your contributions!