Adds a conditional import of the DatetimeAccessor due to a change in the backend API of xarray v0.12.2. Fixes #1077.
Adds a conditional import of the DatetimeAccessor due to a change in the
backend API of xarray v0.12.2. Fixes #1077.

Updated import method per @jthielen's suggestion

ahaberlie/MetPy,dopplershift/MetPy,dopplershift/MetPy,ShawnMurd/MetPy,Unidata/MetPy,ahaberlie/MetPy,Unidata/MetPy