Refactor MultiIndexSet.add_exponents() with inplace

The method add_exponents() of the MultiIndexSet class has been updated with an inplace flag parameter to allow the modification to the underlying exponents to happen either directly in the current instance or as exponents of a new instance (the default behavior). This MR should resolve Issue #117 (closed).

A couple of additional changes:

• The test suite has been extended to test the behavior of adding exponents both inplace and outplace.
• The in-code documentation of add_exponents() has been updated to conform with the Minterpy standards.
• The function core.verification.check_values() has been refactored with improved in-code documentation.

Because the hidden method _new_instance_if_necessary() is not used anymore by add_exponents(), Issue #81 (closed) is also resolved by this MR. The test suite has been extended to check the previously broken behavior.

For this MR, I'm requesting a review from @schrei06 (Thanks a lot!).