Fix gfp computation

[vferat]

As discussed in https://github.com/vferat/pycrostates/issues/160 and https://github.com/vferat/pycrostates/issues/195, introduces changes to the Global Field Power (GFP) computation based on the selected channel type.

When the channel type is eeg, the GFP is now computed using the standard deviation (std). When the channel type is grador mag, the GFP is computed using the root mean square (rms).

[mscheltienne]

This is good to merge? @vferat