add energy system cost subcategories

[robertpietzcker]

• add a "supply" subcategory
• shift "electricity" under this "supply" subcategory
• add a split into "fuel cost", "investments" and "operation and maintenance cost"

[robertpietzcker]

@danielhuppmann the problem persists?

[robertpietzcker]

thanks for finding the missing "unit", I somehow hadn't understood the check result in this way.

now it works - could you please merge, @danielhuppmann ?

[robertpietzcker]

@danielhuppmann @phackstock apologies for bugging you again - it would be great if you could merge this so the models can upload the new variables

[danielhuppmann]

Looks ok

[robertpietzcker]

thanks a lot!