Storage doesn't function just with s_cap.max

[brynpickering]

Currently, creating a technology with the parent 'storage' and simply defining an s_cap.max for it will not work. The constraints c_es_prod_max_rule and c_es_con_max_rule force the storage facility to produce and consume no energy because e_cap.max isn't defined (and is thus automatically reduced to zero).

e_cap is useful to use to allow a maximum flow of energy to/from the storage facility, but it shouldn't be necessary for storage. I'd suggest saying that if e_cap.max is not defined for a storage facility that e_cap and s_cap is matched. I haven't patched and pulled this because it's probably worth discussing first.

[brynpickering]

I'd actually suggest that e_cap.max is defined as a proportion of s_cap. For instance, a battery usually can charge/discharge at about .3 - .5 its capacity.

[brynpickering]

Either e_cap.max or s_cap.max is defined for storage, with c_rate being a constraint parameter used to link the two (s_cap*c_rate = e_cap).