Add a new MultiIndexSet method to make a set of exponents downward-closed.

The method make_downward_closed() of the MultiIndexSet class is now available to make the underlying set of exponents downward-closed. The method fills in all backward indices that are missing from the current set until the whole set becomes downward-closed.

Following the convention adopted by other exponent-altering methods, such as, add_exponents() and make_complete(), the method make_downward_closed() also supports a flag parameter inplace to determine whether the change to the underlying exponents happens in-place or out-place. By default, the change happens out-place (i.e., a new instance is created).

This MR should resolve Issue #123 (closed).

Other changes included in this commit:

• A utility function that implements the routine to make an array of multi-indices downward-closed is added as minterpy.core.utils.make_downward_closed().
• The test suite is extended to test the behavior of the utility function as well as the new method make_complete() of the MultiIndexSet class.
• A new How-To Guide on making a multi-index set downward-closed is added to the docs.