[Enhancement] Include fixed parameters in Batchrunner.get_model_vars_dataframe()

[Corvince]

Hey all,

I really come to like the mesa included Batchrunner. However, I also want to keep track of the fixed parameters I used for the simulation run. Maybe others feel the same so I added a include_fixed option to the Batchrunner.get_model_vars_dataframe() function. Feel free to use my branch or even merge it.

Best
Vincent

[TaylorMutch]

Closing as #423 has been merged.