Remove dependency on netcdf

The only place in the library where we use netcdf is in the tests [here](https://github.com/ClimeTrend/cloudcasting/blob/main/tests/conftest.py#L19-L21). We rely on netcdf to load x and y coordinates which are used to create some sample satellite data for testing.

We could remove this dependency if we converted the test data to another format such as a zipped zarr

Start with tests/conftest.py lines 19-21 to understand how netcdf loads the x and y coordinates for sample satellite data. Evaluate the proposed alternative test-data format and verify that the test suite passes without the netcdf dependency; done means the dependency is no longer required for these tests.

Written by the indexing model from the issue text.