This should, in due time, allow training of groups of units (e.g. 3 archers, 2 spearmen) or a random selection of part of a group of units.
E.g. one can specify a `composition` with two entries Unit A and Unit B. On spawn both units will be spawned, unless `MutuallyExclusive` is true, then a random entry is chosen to spawn.
Yet to do:
** Decide unit on spawn.
- Look at `input.js`.
- Make stuff useful for training optional, so that the cost can also be calculated based on the individual entites to spawn.