Delete unused Cell and Rule Classes

They were used under the old system of every
gridspace being its own cell data structure with its own rules to observe. 

Replaced by the RuleSet class. Cell has vanished in favor
of simple boolean values filling the grid.

lib/src/Cell.dart

@@ -1,6 +0,0 @@
-import 'package:rules_of_living/src/Rule.dart';
-
-class Cell {
-  /// For determining if render updates are necessary in [Grid].render() function
-  bool dirty = false;
-}

lib/src/Rule.dart

@@ -1,5 +0,0 @@
-class Rule {
-  final Function evaluate;
-
-  Rule(this.evaluate);
-}