All implementations of IStrategy should test input cells are not modified

[mmmcaffeine]

Currently we're using bool?[] to represent cells, but passed as IEnumerable<bool?>. It would be possible for an implementation of IStrategyto cast this to the specific type and fiddle with the content. We do not want to allow this. Managing the state of any grid would be the responsibility of any kind ofGameorPuzzle` type, not the strategy itself.

It might make sense to implement this behaviour in StrategyBase. That type could take a copy of the input array and pass that to the template method. That would guarantee inheritors cannot modify the original because they never see it. That way we only have to implement the tests for one type, rather than every implementation.

This might become a non-issue if we implement Line (#1) and Hint (#2); we'd expect to implement those as immutable types so implementations of IStrategy would have a hard time changing them 😉