Feat additional manipulate functions

[Darxo]

These are two minor ideas for the perk group class that can improve submodding, when one of the mods wants to change things done by another.

[LordMidas]

Definitely seems like a good idea to return the tier of the perk when removing from a perk group. I'm not completely convinced on the replacePerk function because how would that work in practice in terms of achieving a goal. If my goal is to replace perk A with perk B in a perk group:

perkGroup.replacePerk(perkA, perkB); 

seems fine but then I want to know whether the perk actually got replaced and if not then I need to add it myself so in fact I still end up doing something like:

local tier = perkGroup.removePerk(perkA);
perkGroup.addPerk(perkB, tier == null ? 4 : tier);

In order to get around this we could add an optional parameter _tier = null to the replacePerk function e.g.:

perkGroup.replacePerk(perkA, perkB, 4);

If you don't pass the tier, then the function only tries to replace the perk and if perkA isn't found, then perkB isn't added. If you pass a tier, then if perkA isn't found then the function adds perkB to the given tier. If perkA is found then the passed _tier is ignored and perkB is added to the same tier at which perkA existed.