regular expressions appear to be working now

(...at least simple ones do, such as "@{atom:re.C[1-2]}")
Regular expressions can now appear in "_coeff" commands as well as in "By Type" angle generation sections (which typically appear in force-field files). More testing is needed. Corresponding updates were made to the manual ("moltemplate_manual.tex"), but the pdf version of the manual has not yet been updated.
An early, crufty version of the "postprocess_transitions.py" file has been added. (It is not yet complete.)