Fixed bug. All messages says "n_combinations is larger than or equal … (#360)

* Fixed bug. All messages says "n_combinations is larger than or equal to 2^m", but all the test only tested for "larger than". I.e., if the user specified n_combinations = 2^m in the call to shapr::explain, the function would not treat it as exact.

* Added script demonstrating the bug that shapr does not enter the exact mode when `n_combinations = 2^m`, before the bugfix.

* Added (tentative) test that checks that shapr enters exact mode when `n_combinations >= 2^m`. Remove the large comment after discussing that with Martin.

* styler

* minor edits to tests

---------

Co-authored-by: Martin <jullum@nr.no>