Added basic skeleton for libmcell framework

This commit adds a first pass implementation of a libmcell API
based on existing functionality. Currently, the API only has a handful
of functions which more or less wrap around existing functionality after
some significant refactoring. Nevertheless, the main mcell driver (mcell.c)
is now fully implemented in terms of the API exposed by libmcell.h and
thus separate from the rest of the code.

The next step will be to provide more fine grained API exposure of
functionality currently jumbled up in the API calls mcell_parse_mdl
mcell_run_simulation. The former should result in detailed control
over setting of simulation properties, addition of molecules and
geometry etc. The later will provide more fine grained runtime control.
Merge branch 'libmcell_api'