Made cell::Cells Copyable and Equalable
Made unit::Unit usable

* Implmented unit::Unit.contains
* Added unit::Unit.get_cells
* Implemented fmt::Show for unit::Unit
* Made unit::Unit public
Added get_<unit> functions to board::Board

These functions return a new unit::Unit type based on the function.

* get_row -> Unit::Row
* get_column -> Unit::Column
* get_grid -> Unit::Grid