techs.yml constraints for bat_e

[holgerteichgraeber]

@YoungFaithful What is the definition of efficiency in the constraint for bat_e?

I see that bat_in and bat_out both have efficiencies, which I assume to be charge and discharge efficiencies.

[holgerteichgraeber]

storage: efficiencies are in efficiency per month https://youngfaithful.github.io/CapacityExpansion.jl/dev/README_GER_18/

[holgerteichgraeber]

@YoungFaithful adding comments in yml file.