Automatically select sample rate

Created by: eendebakpt

When plotting with the default sample rate (1e9) python can crash because of the large amount of memory used. For a CSD scan with 100 ms scan time a 1e9 samplerate results in 1e8 elements for each channel.

In this PR we automatically select the samplerate such that the number of points plotted is below 1e5 (unless the user specifies the samplerate manually). @sldesnoo-Delft