Fix typos in comments
MAIs: add utility functions for storing information in AI's self.data

These provide a simple and consistent method of storing variables
inside [micro_ai] tags in the AI's persistent self.data variable. This
is needed for fixing a bug with variables stored there from a previous
MAI causing problems with a subsequent MAI
Micro AIs: check for existence of [micro_ai] tags in self.data when...

… setting up an MAI. Ideally, we would delete such previous occurrences
of [micro_ai] tags in the AI's self.data variable. However, the MAI can
be changed while it is not the AI's turn, when this is not possible. So
instead, we check for the existence of such tags and make sure we are
using a different ai_id.
Micro AIs: use the new functions for storing information in self.data

… so that it is done in a consistent way and conflicts can be avoided
when setting up a subsequent MAI.