Implement __eq__ dunder method for MultiIndexSet

Currently, checking equality between two MultiIndexSet instances falls back to checking whether the two instances share the same identity (basically is). What we want, however, checking whether two MultiIndexSet instances have the same value:

>>> import minterpy as mp
>>> a = mp.MultiIndexSet.from_degree(2, 3, 1)
>>> b = mp.MultiIndexSet.from_degree(2, 3, 1)
>>> a is b
False  # As it should be; they are not the same object
>>> a == b
False  # Should have been True

To check the equality of value between two MultiIndexSet instances, we need a custom __eq__() dunder method. I propose implementing this method and define that two MultiIndexSet instances are equal in value iff:

• The sets of exponents are the same
• The lp_degree properties are the same

Because the above properties tell us everything about a MultiIndexSet instance. Other important properties like spatial_dimension and polynomial_degree are actually inferred quantities from the given two above properties; so there's no need of explicitly checking them.

One possible complication from the implementation is checking the equality between two lp_degree's; they are stored as float in the instance. We'll use math.isclose but who knows what kind of edge cases that may arise from comparing floats with representation errors... The exponents are stored as NumPy integers so checking the equality won't be complicated.