[electrophysoiology_browser] MEG-compatible channel visualizer

[MaximeBICMTL]

The electrophysiology visualizer uses EEG-specific constants like its unit and range (see the file vector/index.tsx). Unfortunately, those are not compatible with MEG that uses different units on different orders of magnitude. With the current constants, MEG can be displayed in the visualizer but the channels are all straight lines. By changing the STATIC_SERIES_RANGE to [-0.00000005, 0.00000005] I got the following screenshot:

Obviously this is just a proof-of-concept.
We also see that in MEG, some channels have large amplitude differences (is it the same in EEG ?). So that may need to be taken into account.