GUI: Enhancements to improve tutorials

[ntolley]

Going through the tutorials has revealed some modifications that would make using the GUI much smoother for users:

• Add global smoothing and scaling option (temporarily set default smoothing to zero (right now these are set manually for every new plot you make)
• Allow setting min/max freq on PSD
• Disable spectrogram specific parameters when other plots are selected
• Add E/I synaptic gains button (already implemented in API: Synaptic gains #885)
• Add ability to set simulation data to None so that you can just load in and plot a data file

For discussion

• Disable simulation data in Data to compare box, you can plot multiple simulations by hitting the Add plot button multiple times

[dylansdaniels]

See #914 for proposed PSD fix

[dylansdaniels]

• add observer to monitor change in tstop in Simulation tab to ensure simulation length can be correctly specified for rhythmic, poisson, and tonic external drives

[gtdang]

#918 for GUI synaptic gains implementation