Define an empty MultiIndexSet instance.

An empty MultiIndexSet instance has an empty array as its exponents. It can be created by passing an empty two-dimensional array to the default constructor. Such a set has an lp-degree that is defined during construction; its spatial dimension, however, may be 0.

The properties of such a set are as follows:

• exponents: a two-dimensional empty array as specified.
• lp_degree: as specified during construction (float).
• poly_degree: None (it cannot be computed).
• spatial_dimension: according to the value of the second dimension of the given empty array (may be 0, int).
• is_complete: False a priori.
• is_downward_closed: False a priori.
• __len__(): 0 (i.e., zero cardinality).

Empty sets are defined to support further set operations involving MultiIndexSet. Such a set cannot be used to construct polynomials or Grid instances.

This MR should resolve Issue #132 (closed).

Several additional changes:

• A safeguard is introduced to the constructors of polynomials, Grid, and OrdinaryRegression not to accept an empty set.
• The test suite for MultiIndexSet is refactored to uniformly use class-based grouping for organization purposes.
• The same test suite is updated with testing the behavior of all methods involving an empty set.
• The test suites for polynomials, Grid (newly added), and OrdinaryRegression are updated with testing the construction using an empty set.