[FEA] Support Generalized Adjustment Criterion for Estimation plus Add Example Notebook

[nparent1]

Adds support for the generalized adjustment estimands at the estimation stage. The PR primarily does two things:

• Abstracts the get_backdoor() method of the IdentifiedEstimand class to instead be a get_adjustment_set() method. Then if the identifier_method is specified as 'general_adjustment', the get_adjustment_set() method will return the general adjustment set (rather than explicitly a backdoor set). Then throughout the repo where get_backdoor_set() was called for estimation purposes, we instead call get_adjustment_set(). Note that because we have get_adjustment_set() default to the backdoor set, the overall behavior is the same as before, unless a user explicitly sets general_adjustment as the identifier_method.
• Adds various tests and test utils. In particular, adds support for specifying explicit graphs in the estimation tests, for more specific tests of the general_adjustment criterion.

An example notebook is also added, dowhy_generalized_covariate_adjustment_estimation_example.ipynb

This is a follow-on PR to #1292

Issue Ref: #402

[nparent1]

This will need to be rebased once #1292 merges

[nparent1]

Minor bug fix here - taking the absolute value, both here and a few lines down.

Previously the test util would return true if (error < true_ate * self._error_tolerance) which only makes sense if the error and true_ate are in absolute value (otherwise a negative error would always be accepted no matter how far from zero)

[nparent1]

This is modeled after the graph objects used in the identification test suites (see base.by in the identification test folder).

I added this so that more specific graphs could be tested as needed (which is particularly relevant for testing some of the more involved graphs which really differentiate the general adjustment criterion from the backdoor criterion)

[nparent1]

This is the core change. IdentifiedEstimand now has a generic get_adjustment_set method which routes to one of get_backdoor_variables or get_general_adjustment_variables.

The wrapper method get_adjustment_set should thus be called instead of get_adjustment_set or get_general_adjustment_variables throughout the codebase.

[nparent1]

@amit-sharma - this is a follow on to #1292 to add support for general adjustment for estimation. It is ready for review