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)
• [x] Allow setting min/max freq on PSD
• [ ] Disable spectrogram specific parameters when other plots are selected
• [x] Add E/I synaptic gains button (already implemented in #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