Add technology-specific variables for techno-economic parameters

[danielhuppmann]

This PR adds variables for technology-specific parameters. The * is a wildcard so that any technology can be added to a variable, e.g.

Capital Cost|Electricity|Biomass|1
Capital Cost|Electricity|Biomass|CCGT

can be used in reporting.

Requires IAMconsortium/nomenclature#432 to validate the units as well as the variable names.

[masahirosugiyama]

@danielhuppmann Thanks. It looks good. I have a minor suggestion. We have this:

description: Capital cost of a newly installed plant to generate electricity
  from {Electricity Source} for a specific technology

Can we change "a specific technology" to "a specific technology or a resource grade" or something like that?

[danielhuppmann]

Can we change "a specific technology" to "a specific technology or a resource grade" or something like that?

@masahirosugiyama, not implementing this suggestion because resource grade doesn't apply to all electricity technologies or fuels.

[danielhuppmann]

Merging to send this out to modeling teams today.