Compile-time RHS evaluation should have a pretty print option

[maxpkatz]

For all of the new-style networks, the actual_network.H file specifies the metadata that determine each reaction, but it does so in an implicit form. It would be nice to have a pretty-print option that writes out the explicit form of the RHS (and Jacobian) as it would appear if we had written out the C++ explicitly, i.e. to show the full expression ydot(He4) = ....