more datamodel verification

[lea-hagen]

There is currently only limited verification of the datamodel parameters (verify_params, which will be incorporated into the datamodel class in #326).

The current checks:

• logt/av/rv/fA: formatted as [min, max, step_size] and within proper boundaries
• z (metallicity): list of floats within proper boundaries
• obsfile/astfile: strings

There are a lot of other things that should probably be checked, the most basic being that all required parameters are present.