power update

I’m still not sure why 4.0hz is the interpolation frequency used in
like every example ever, but we’re going with it. along with gHRV’s
values for elf, elf, lf, and hf. unfortunately, i’m not sure how to get
standard error for these values, as i only have the resulting PSD
arrays to work with.

currently, i’m working on finding the area under the curve at these
ranges so I can report them.

#16683064947985