Add a composite to visualise low level moisure using the ratio between
0.91 µm and 0.86 µm.  Experimental.

First attempt to add a colorised version of the low level moisture
composite.  Work in progress as the actual colors are not linear.

Fix syntax error in enhancements/generic.yaml by removing
an erroneous hyphen-minus.

Rename the low level moisture to include essl in the name.
Another attempt at the colorised ESSL low level moisture.
It still looks wrong, continuing to debug.

There is a bug in the simulated FCI test data for the 0.865 µm channel.
Add a correction in the enhancement colorisation to correct for this
bug.

For the ESSL moisture colorisation, add to the documentation the
equations used to calculate the colours.

Add the readthedocs mathjax plugin.

In test_all_datasets_one_reader, update expected number of composites to
account for new composites in visir.yaml.

Fix the daytime versions of the low level moisture composites.
Remove the redundant MODIS low level moisture composite.