Add technology parameters for storage and other electricity variables

[robertpietzcker]

Added a number of variables required for an ECEMF look at storage representation in IAMs and ESMs

• added storage charge and discharge as part of Secondary Energy|Electricity
• added capital costs for storage - both aggregate, for power and for reservoir
• added conversion efficiency entries for electricity technologies
• added roundtrip efficiency entries for electricity storage technologies
• added fixed costs for storage technologies

if partial overlap with existing variables is a problem, we might instead shift this to the ECEMF-internal additions to the openENTRANCE template, if I understood @danielhuppmann correctly?

[robertpietzcker]

pushing this up again @danielhuppmann

[danielhuppmann]

Looks good, thanks!