Support for inline local functions

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Currently, NFsim does not allow local functions to be defined "inline" in the 
reaction rules block. If one attempts to do this the following error message is 
thrown:

    [------Error!! call a local function through a composite function always!
DOR rxn should never directly call a local function.

It's not clear to me why this is not allowed, although there may be a good 
reason. If so, perhaps we can modify writeXML on the BNG side to add the 
additional layer of separation between the local function definition and the 
rate law that NFsim requires.

Original issue reported on code.google.com by lh64@cornell.edu on 6 Jun 2013 at 7:13