The method '__repr__()' of MultiIndexSet should provide an unambiguous string representation of the instance

Following Python best-practice, the string information provided by __repr__() can, at least in principle, be used to recreate the instance. It is targeted towards developers to get as much useful information about the object.

The string provided by __str__() on the other hand is targeted towards users; it provides user-friendly information.

Currently, only __repr__() is implemented for the MultiIndexSet class while the implementation of __str__() use that of __repr__(). In other words, they produce the same string.

>>> import minterpy as mp
>>> mi  # interpreter calls __repr__()
MultiIndexSet
[[0 0 0]     
 [1 0 0]     
 [2 0 0]     
 [0 1 0]     
 [1 1 0]     
 [0 2 0]     
 [0 0 1]     
 [1 0 1]     
 [0 1 1]     
 [0 0 2]]
>>> print(mi)  # print() calls __str__()
MultiIndexSet
[[0 0 0]     
 [1 0 0]     
 [2 0 0]     
 [0 1 0]     
 [1 1 0]     
 [0 2 0]     
 [0 0 1]     
 [1 0 1]     
 [0 1 1]     
 [0 0 2]]

I propose to modify the __repr__() of MultiIndexSet to include at least the information regarding lp_degree as the construction of an instance of MultiIndexSet class requires at least the set of exponents and the lp-degree. The implementation of __str__() may also be modified to include the information regarding spatial dimension, poly. degree, and lp-degree.

This issue is created as part of the refactoring of the MultiIndexSet class (Issue #99 (closed)).