Support cftime units natively in xr_linregress, linear_trend and xr_detrend

[jbusecke]

Currently these fail if a coordinate containing cfdatetime is passed. Is there a way to deal with this natively? This would be very interesting for irregularly spaced data.

linear_trend for instance is just assuming that the data is regularly spaced and will work on logical indicies of the dim that is passed. But it would be nice if xr_linregress could deal with these more complicated cases.