Support ChangeModel

[martinvonk]

ChangeModel is currently not present in this list:

pastastore/pastastore/validator.py

Lines 296 to 304 in 96fb0f0

	supported_stressmodels = [
	"StressModel",
	"RechargeModel",
	"WellModel",
	"TarsoModel",
	"Constant",
	"LinearTrend",
	"StepModel",
	]