various additions and improvements to data

[tylerflex]

• fixed convert.py for time monitors
• added sim_data.plot_field() for structure overlay.
• Tidy3dDataArray with .abs property
• sim_data[mon_name] returns either xr.Dataset or Tidy3dDataArray
• remove inf-based default limits on medium.frequency_range (None means all accepted)
• make .parse_xyz_kwargs a public method.
• changed default plotting viz settings.