added detrending (#13)
added detrending (#13)

* added detrending

* slight updates we discussed

including convert_datetime option, removing start_shift

* Some refactoring and updated docstring

I tried to make this as general as possible and added an option to pass precomputed trend data.

* Fixed typo

* Fixed another typo in xr_detrend

jbusecke/xarrayutils