Option to set time units for ROI trace x-axis in GUI

[generalciao]

The GUI plot of selected ROI trace(s) uses frame number as the x-axis units (and label). It would be useful to (have the option to) show time in seconds, rather than in frames. One use case is having a variety of frame rates depending on the particular acquisition scenario; the other is when exploring new data, to easily see the time scale of events and activity patterns.

Since suite2p's algorithms require frame rate and indicator time constant inputs, I believe the time-base of the traces is known "under the hood", and so hopefully implementing this feature would (simply?) require passing a different variable to the plot function and rendering an axis label to indicate the units. However, not all users / cases may want to display seconds instead of frames, so probably an additional GUI control is needed to toggle between both modes.