add CAL line frequencies to config file, and calculate DARM EPICS records explicitly

The main goal here is to be able to access the values of the calculated EPICS values to be written to the front end directly from the DARMModel. In order to do that we need the CAL line frequencies, which means they need to be in the config file (until we can just read them from the front end directly (which should be possible)).

We also clean up some of the code by moving some of the generic functionality into the util module (writing a dict to file, writing out EPICS records).

Closes #107 (closed)