Add support for land cover data

[BSchilperoort]

This PR adds support for the CCI land cover dataset.

The following have been added:

• [x] Download script
• [x] Notebook to open and read some data from the netCDF file
• [x] Land cover translation table
• [x] Read routine, with support for time-dependent land cover
• [x] Checks & tests

I decided to remove the dem and canopy height text files, as those have been integrated into the package (as compressed txt files).

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