feature to 'explicitly' require a certain rule

[mr-highball]

when working to implement #3 it was clear that there wasn't a way to enforce the output of a graph to make sense when dealing with a complex rule set. for example, if certain elements (like a wall mid section) can only be present in between other components or directly beside something.

AC's

• when adding a rule, allow for an optional boolean parameter ARequireRule
  • default this parameter to False
• when ARequireRule is True the following should take place
  • rule added to the internal rules collection with this flag
  • TGraph.DoValidate() needs to be altered to handle the Required flag
  • when True Force the constraints of a rule with Required to be present
• when ARequireRule is False the default behavior for wfc should remain
  • rule added to the internal rules collection with this flag

[mr-highball]

will be closed when #3 merged to master