`mt_history_[grid]` only stored once per grid

For mt_history_[grid_type] we only have one entry per grid type as a result we can only record a single mass transfer history type. If one wants to enter the same grid multiple times, this cannot be stored.

Additional info edit:
The mt_history_HMS_HMS only holds a single mass transfer event, for example: "stable-reverse mass transfer". However, it might have experienced a phase of normal "stable mass transfer" beforehand, which is not recorded.
As such, mt_histoy_HMS_HM contains only: "stable-reverse mass transfer", while we might want it to contain both: ["stable mass transfer", "stable-reverse mass transfer"]