Report which parts of a region have not been filled in yet #43

Background

A cell with a weight of zero was never written to. Dividing by it gives nonsense rather than an error, so nothing catches it on its own.

Description

Create a function that says whether a region has been completely filled in. Reading a grid that still has unfilled cells should fail with a clear message rather than hand back the nonsense.

Testing:

• Assert a region nothing has been written to reports as not filled in
• Assert a region fully covered by windows reports as filled in
• Assert a region with a gap in the middle reports as not filled in and raises an error when read