added command caching system (ported from cortex-forge):
- certain commands are now "cached" for each unit in global data table, so each unit has a command history, which can be used for saving units, undoing commands, copying unit visuals
- commands to be cached require a case in the caching system in unit.galaxy along with the cache update function in the command

also added unit death data table cleanup and proper unit creation unit ID and data table initialization