Modify PySB to detect dangling bonds

[lh64]

Currently, patterns like A(b=1) % B(a=2) are allowed in PySB. These are examples of dangling bonds that should be illegal, and formally are in BNGL. Unfortunately, however, BNG does not currently have any code for detecting dangling bonds, so they are actually legal syntax in practice. Therefore, if a PySB rule is accidentally created with dangling bonds (which has happened, see Issue #9 ) there is nothing currently to detect and prevent this. It probably makes sense for us to implement a dangling bond detector at the PySB level since it's unclear when (if ever) this issue will be resolved in the BNG code.

[alubbock]

@lh64 Detecting dangling bonds should be straightforward, but should they cause a Python warning, or an error?

[lh64]

It's illegal syntax, so I think an error.

[alubbock]

Implemented in pysb#328