[MRG] Remove nulled drives during convert to hierarchical json function

[gtdang]

Updated the hierarchical json conversion script to prune nulled drives. Legacy param files had placeholders for several drives. If unused, values out of typical ranges were specified to null those drives. Typically this occurs when a start time is set greater than the stop time or if weights are set it negatives or 0s. The hierarchical json format will not include drives with bad parameters.

Summary of Changes:

• Added logic to remove drives that did not have at least 1 positive weight or had invalid tstop and tstart values.
• Removed _modeltemplate argument from convert_to_json and associated tests - After discussion that we are using this function internally to convert tutorial files. And they will all be the jones2009 model.

Questions

1. Are the filters for nulled drives correct/complete?
   • Check for start < stop
   • Checks nmda and amps weights for at least one >0 value

[gtdang]

@ntolley Can you check to make sure the logic I'm using to identify nulled drives is correct? There's a check on the start/stop time. And for the weights, it's checking for at least one positive ampa and nmda weight. Could there be other weight conditions we should exclude or include?