Implement equality check for Grid instances

A __eq__ method has been added to the Grid class to check for equality between two instances based on their multi-index sets and generating points. The equality check is exact in nature as the generating points are of floating type; no tolerance is specified.

Tests for this equality check have also been added to ensure it functions correctly, handling cases of identical instances, instances with equal values, and instances with different values and inconsistent types.

This MR should resolve Issue #137 (closed).