Update energy service variables & Add Useful Energy

[jkikstra]

For ScenarioMIP analysis, per discussion in #270 and also discussion with @vruijven @christophbertram @gunnar-pik @volker-krey

[jkikstra]

Not sure what exactly is causing the 'illegal character' error:

=========================== short test summary info ============================
FAILED tests/test_legacy.py::test_legacy_variables - ValueError: Collected 2 errors:
  1. Unexpected character in variable: 'Production|Iron and Steel'. Check for illegal characters and/or if tags were spelled correctly.
  2. Unexpected character in variable: 'Production|Non-Metallic Minerals'. Check for illegal characters and/or if tags were spelled correctly.
FAILED tests/test_reserved_terms.py::test_variable_ops_as_square_brackets - ValueError: Collected 2 errors:
  1. Unexpected character in variable: 'Production|Iron and Steel'. Check for illegal characters and/or if tags were spelled correctly.
  2. Unexpected character in variable: 'Production|Non-Metallic Minerals'. Check for illegal characters and/or if tags were spelled correctly.
========================= 2 failed, 1 warning in 6.05s =========================

[danielhuppmann]

This replacement doesn't work (if the tag isn't used in the variable name), hence the illegal-character error.

[jkikstra]

OK, so if I want to add a top-level variable like this & add the list of components, would there be another way, or would I always have to list them all, or just not list the components?

[danielhuppmann]

I would advise against listing components if these are all sub-categories. This is more confusing than helpful.

[jkikstra]

OK, removed.