Training low level mercs

Implement a way to keep your 2nd/3rd tier mercs to level up outside of fights.

As an example, Battle Brothers has training halls to boost EXP gain, for a price.

BB allows far more mercs to be fielded, so even weak mercs can be supported and less of a drain on the party. However, given we only have 5 party members at a time in Iron Oath, it would be fairly suicidal to bring a level 3 with your level 8 squad. I find myself unwilling to train any of my lower level mercs in this game.