Documentation/example enhancements

[holgerteichgraeber]

As I am trying to use CapacityExpansion, I'll make some notes here of things where better examples/documentation would help:

• OptVariable, and how to use get_cep_variables() / get_cep_slack_variables() . It is not intuitively clear how to get the values from the fields. Also, what is SLACK? I assume it is just LL for every hour?
• should sv["LL"].data and sv["LE"].data be floats instead of array{float}?
• explain that they can be indexed, e.g. : sv["SLACK"]["el",1,1,"germany"] (this is currently not in the documentation of OptVariable)
• add an example with co2_limit. Mention default in the documentatoin.
• Explain the operations optimization function (that if you pass it config, it automatically takes some arguments. list which ones. In my use case, I got an error when I redefined co2_limit. That should happen and is great. Should just be documented.)
• Mention that Gurobi is much faster than Clp and should be preferred if in an academic setting
• show an example of how to configure gurobi in docs (e.g. outputflag=0)
• run_opt documentation: for each option, state the default
• transmission in run_opt(): Does this include transmission as variables? How do I set fixed transmission infrastructure (existing), but without transmission as a variable?
• in run_opt(): if transmission=true, does that mean I allow transmission as a variable? and if transmission=false, I do not allow transmission as a variable? Does false mean that no energy exchange between different nodes can happen?

[holgerteichgraeber]

Another addition for the documentation: What is the unit of LE, is it kg CO2 or t CO2? It just says LCA category unit, but there seems no documentation on what that is for CO2.

[YoungFaithful]

Addressed in #28