Clean up the implementation of the new Aronnax driver and environs

[axch]

There are several things we can do once #101 is merged that will make the resulting code base simpler. We should choose whether and when to.

• [ ] Perhaps replace the driver.data_types dictionary with a dictionary in core, directly holding the generator functions themselves, without indirecting through a string and a chain of if statements for interpreting it.
• [ ] Perhaps move core.fortran_file to the utils module.
• [ ] Perhaps simplify the Fortran core, the parameters.in file, and the inside of the driver by hard-coding a naming convention for the raw input files, since they are now generated anyway.
  • Con: Anyone who wishes to isolate the Fortran core and wrap it differently will have a less flexible object to work with.