Add detrend option

[NevPalmer]

Add a detrend option. ie calculate a simple linear regression (trend line) and subtract this from the data. This was an option with the Java applet and is useful to see any deviations from the general trend (eg slow slip events) when the data is otherwise trending steeply. An example of where this would be useful is station GISB where the average velocity east is almost zero so the variations in the east component are obvious (http://fits.geonet.org.nz/plot?networkID=LI&siteID=GISB&typeID=e). Whereas, there is a significant velocity north which somewhat masks the variations in this component (http://fits.geonet.org.nz/plot?networkID=LI&siteID=GISB&typeID=n)). Detrending would highlight the variations (slow slips) more clearly.

[elidana]

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.

[rumachan]

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.

[elidana]

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