Loading and subsetting data speed up

[b-barton]

Description of plan:
Incorporate Xarray preprocessing for subsetting into COAsT. This could be done by loading data using xr.open_mfdataset() and passing it a subsetting partial function in the "preprocess" keyword.

AC:

• Loading CANARI data is sped up
  

• New example dataset is made available
  

• A python notebook describing how to play with CANARI data is written
  

• A unit test for new code is added.