First set of refactoring for reaction API.
More refactoring of code for creating reactions.
More refactoring of reaction addition code.
More refactoring for reaction rate code.
More refactoring for reaction code.
Added initial refactored code for prepare reactions.
Refactored more reaction code.
More reaction code refactoring.
More reaction code refactoring.
More reaction API refactoring.
Removed printf statements so testsuite won't fail.
Refactored more reaction creation code.
Fixed memory leaks introduced during refactoring.
Fixed a number of small bugs uncovered by testsuite.
Merge branch 'reaction_api'

This commit adds an initial API for adding reactions and includes a
significant amount of refactored code. The parser now calls
mcell_add_reactions in libmcell.h to create new reactions.

In addition, all code previously in prepare_reactions as part of
mdlparse_util.c was moved out of the parser into core MCell.

Currently, the API for mcell_create_species and mcell_add_reactions
don't quite match. However, once they're synchronized we should
be able to create species and add reactions between them programmatically
via libmcell from within C.