Change start_symbol to start (to be more consistent and easier to type).
Allow an HRG/FGG to have an initially unspecified start symbol (closes #155). In #107 it was mentioned that this breaks the invariant that an HRG/FGG always has one start symbol, but invariants are impossible to enforce and I think convenience probably wins.
This should be a pretty easy one:
start_symboltostart(to be more consistent and easier to type).