Add documentation on the land-use emulator

[OFR-IIASA]

In order to review, please make a local build of the documentation and read /message_doc/_build/html/land_use/emulator.html

[behnam-zakeri]

This sentence "At the core, these state that the linear combination of land-use pathways must be equal to 1. Therefore, separately" and the following discussion is not clear. Is this the sum of land shares from a total land available?

[khaeru]

Per discussion in Slack, the 2020 documentation will be released now that #51 is in. This PR should be (a) rebased, (b) updated with GLOBIOM documentation from the colleagues who will supply it, and (c) merged in advance of a 2021 release.

[OFR-IIASA]

A few comments from a conversation on slack (from Behnam, Oct. 13, 2020) to improve the documentation of the emulator:

• the legend and labels of some figures can be improved by a more suitable size, also for the figure captions adding more information on each sub-plot when there are multiple figures. (e.g., figure (above) shows..., while below shows...).
• does it make sense to add numbers to equations (Eq. 1, ...)? Then, it's easier to refer to them in the text.
• the costs and prices are in $2005. It may be worth to update this as we are in 2020 and it may be difficult to back-calculate the numbers.

[OFR-IIASA]

@behnam-zakeri I have made changes to address the last set of comments:

• Adjusting all figures and describing the panels in the text
• Switching all units to $2010
• Adding formula references

[khaeru]

Compiles with no warnings. Thanks @OFR-IIASA!