Switching AK5 to AK4

This is a large PR (blame the JME conveners) : 
   * Switches AK5 to AK4
   * Does NOT include AK5 as per request of Slava, but the intention is to put it back for posterity
   * Switched out IC5 (it's about time, five years later)

This includes the tags for other features already integrated as many conflicts were expected (and found) : 
   * Tau updates from Pavel (https://github.com/cms-sw/cmssw/pull/1817)
   * Switching KT6 rho to GBME rho (https://github.com/cms-sw/cmssw/pull/1804)

I ran workflow 25.0 and this worked for me, but it's very difficult to get all of the dependencies.