Expose choice when there are multiple adjustment sets

[christopher-wild]

Currently when multiple viable minimal_adjustment_sets are found, the smallest one is chosen programmatically.

There are reasons however for the user to consider another minimal_adjustment_set more suitable and may wish to have a choice between the different adjustment sets. One reason for example is there could be much better data for nodes of one adjustment set over another.

After a discussion with @jmafoster1 and @AndrewC19 on 25/10/22, it may be most appropriate to add two new parameters to the identification function or to the execute_test_suites function (to be decided).

The first parameter would be a flag to indicate that the code should be halted when multiple adjustment sets are found and displaying the multiple minimal_adjustment_sets. This would give the user chance to explore and decide which adjustment set is most appropriate.

The second parameter would be a mechanism for pre-selecting an adjustment set if the user already knew from previous executions which set was most appropriate.

[christopher-wild]

also look at #58