Add detrend option about fits HOT 3 OPEN

Still relevant for GPS/GNSS time series, even more if that feature is useful also to use volcanic data.
Might be a development of stats endpoint.

from fits.

My view is similar to that I expressed here #65. We need a wider, strategic discussion on what capability we want attached to http://fits.geonet.org.nz/plot? and what might be better served by an alternative program.

from fits.

feature requested again by NevP (and I know other FITS users that would love that)

• The ability to detrend (subtract an average slope from) the timelines. This is really useful when looking at transient changes when the slope/velocity is steep and any transient offsets get lost due to scale.

I am including this here even though that feature might be developed outside of FITS

from fits.