DoubleML 0.9.0

• Release highlight: Average potential outcomes for multiple discrete treatments
  via DoubleMLAPO and DoubleMLAPOS classes (proposed by Apoorva Lal) #245 #250

• Update User Guide and Example Gallery #188 #195

• Add sensitivity analysis to DoubleMLFramework #249

• Maintenance package #264 #265 #266

• Maintenance documentation #182 #184 #186 #193 #194 #196 #197