Implement Union Operation for Grid Instances

The union operation between Grid instances via the | operator is now implemented. The result of such an operation is a new Grid instance whose multi-index set is the union of the index sets of the two operands.

Additionally:

• The test suite has been updated to cover the new implementation.
• A new How-To Guide demonstrating the new feature is added to the docs.

This MR should resolve Issue #155 (closed) and part of the refactoring and update of the Grid class (see Issue #135).