Document undocumented keywords and functionality

[dvalters]

Perusing through: https://bitbucket.org/ncollier/ilamb/src/56101d9468223f12e82767af6a097b9fae6153ab/demo/ilamb.cfg?fileviewer=file-view-default#lines-101

There are quite a few keywords in the config file that indicate extra functionality which is not explicitly documented or covered in the tutorials:

• cmap - sets the colourbar palette from the standard matplotlib list.

• weight (already documented) - sets the skill score weighting for a dataset or variable

• alternative_vars

• mass_weighting

• skip_iav

• ctype

• skip_taylor

• skip_rmse

• table_unit

• plot_unit

• limit_type

• land

• y0

• y1

• Teps

• dmax

• space_mean