Remove support for Attribute change via DPO

[kris-brown]

This addresses #76

This restricts the kinds of rules one may create in order to have better-defined behavior:

1. Left legs of DPO (and SPO) rules must be monic, including attribute variables
2. Variables in $R$ are exclusively either a. in the image of $I\rightarrow R$ or b. assigned an expression to be evaluated at runtime (or c. freevar=true, in which case AttrVars will appear in the result)

Variables in $R$ which are in the image of $I \rightarrow R$ have a unique concrete value associated with them via the match morphism $L \rightarrow G$: that value is preserved in the rewrite.

Extra work now is needed in case one wants to change the value of an attribute. E.g. one can delete the object and recreate it for the time being.