impossible value handling

[Elton Law]

To fix things like negative counts and enforce range limits.

Ways of handling:

• Return list of flagged coordinates
• Coercion function of form
  • def coerce_value(matrix, row_index, column_index) -> value
  • def coerce_value(row, column_index) -> value (can be defined in terms of the above)
• Throw exception

Needed as part of #68