Draft: pyicon_sparse module for performant vector calculus

The following merge request adds a module called pyicon_sparse. The module replicates some of the functionality of the pyicon_calculate_xr module but using sparse matrices under the hood to produce more performant functions, particularly when working with large datasets. See the docstring of the file pyicon/pyicon_sparse.py for more details.

To summarise the changes:

1. New module added, and appropriate changes made to __init__.py
2. sparse added to dependencies
3. Documentation added for new module, including examples. The test fixtures were refactored to enable their use in the examples.
4. Tests added for new module. This involved rewriting/refactoring some of the tests. The test grid now matches the grid of the test model output.