Storage mapping sets do not consider `mode` of operation of technologies

[behnam-zakeri]

The storage equations in message_ix work based on a mapping of charger-discharge technologies to their respective storage device, through the set map_tec_storage. However, this mapping set is defined only with technologies names, not considering the mode of operation. As such, if a storage technology has more than one mode of operation, this would create an issue. Also, including the mode of operation would make it possible to specify a single technology as both charger and discharger, but operating in different modes (like turbo-machinery section in a pumped hydro storage). Therefore, it is desirable to change the indexes of this mapping set from the existing ["node", "technology", "storage_tec", "commodity", "level"] to an enhanced one as ["node", "technology", "mode", "storage_tec", "storage_mode", "commodity", "level"]. However, this is not straightforward at the moment because of the re-initializing issue mentioned in #631.