Refactor the factory methods of the Grid class.

The factory methods of the Grid class have been refactored. Specifically:

• from_generator() is renamed to from_function() to avoid confusion with the Python's "generator".
• from_degree() is introduced to directly create a grid that corresponds to a complete multi-index set without the need to separately create an instance of MultiIndexSet first.
• from_points() is introduced to directly create a grid with a given array of generating points.
• from_value_set() is refactored to only accepts one-dimensional array of two-dimensional array with a single column/row.

Additionally:

• The corresponding tests are updated to reflect these changes.
• Several How-To Guides regarding the construction of Grid instances are introduced in the docs.

This MR should resolve Issue #149 (closed) and part of the refactoring of the Grid class (see Issue #135).