Added Section functionality. Overhaul required.

The Matrix class will likely be removed and saved somewhere else;
currently, everything about a Sudoku object needs to be different in
some small way from a Matrix object, so there's no longer a need for any
hierarchy.
Added a Sector Dictionary

This "sector dictionary" is what a dictionary of all 81 values with
respect to their sectors and sector positions would look like. In case
it's to be used for sector lookups or what have you.