Support `xarray.DataArray` in `resample` function

[geek-yang]

As a user, I want to use resample function in s2s.time module to upscale my xarray type data.

[BSchilperoort]

I have started on implementing xarray data (both DataArray as well Dataset) for the resample method. However currently the resampled data does not have the lag or anchor year as actual coordinates.

To fully implement xarray objects the following steps have to still be done;

• Reshape the output Dataset to have the lag and anchor_year variables as dimensions/coordinates instead.
• Ensure that the original coordinates of the input data are preserved (e.g. latitude, longitude) to the output Dataset without modification..

[BSchilperoort]

Implemented in #28