Simplify XP and morph

* the relative XP of fighting units now has no effect on their XP gains - previously experienced units were treated as more costly for that purpose
* each rank is now making cost exactly once, ie. bringing from 100% to 0 and scoring the last hit - previously varied based on cost, with Flea having to make x1.6 cost per rank, Bantha (currently most expensive morpher) x0.5,  Starlight x0.04.
* the unit's lifetime XP is preserved so that it's the same cost-wise (ie. gets scaled by relative cost) - previously the XP was always halved regardless of relative costs and could be capped depending on further morph options

Effects:
* morph requirements are clear
* the rank bars now always say how many times the unit made cost during lifetime, regardless of the unit type and its morph history
* chain morphs become possible for really experienced units