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