Refactor MultiIndexSet.expand_dim() with inplace

The instance method expand_dim() of the MultiIndexSet class has been updated with an inplace flag parameter (thus conforming with Issues #103 (closed) and #117 (closed)) to allow the modification to the underlying exponents to happen either directly in the current instance or as exponents of a new instance (this is now the default behavior). This MR should resolve Issue #118 (closed).

Several additional changes:

• Because the default behavior of calling the method is now changed (outplace instead inplace), the method expand_dim() of the MultivariatePolynomialSingleABC class has also been updated such that the modification to the multi-index set happens outplace.
• The test suite has been extended to test the behavior of expanding the dimension of an exponent array.
• The in-code documentation of expand_dim() has been updated to conform with the Minterpy standards.
• The function core.utils.expand_dim() has been refactored and its in-code documentation updated.
• The test suite now also includes tests related to calling the utility function core.utils.expand_dim() which previously was not tested directly.