Force dispersion tests with re.form = NULL

[florianhartig]

From a bit of testing, it seems that the dispersion test is a lot more sensitive with re.form = NULL, i.e. with conditional simulations. If that is so (to be confirmed), maybe a conditional simulation (if possible) should be forced?

data = createData(1000, overdispersion = 0.3)

fit <- glmer(observedResponse ~ Environment1  + (1|group) , 
              family="poisson", data = data)

res <- simulateResiduals(fit , n=1000 , plot=T)
testDispersion(res)

res <- simulateResiduals(fit , n=1000 , plot=T, re.form = NULL)
testDispersion(res)

[florianhartig]

I have added some explanations in the help. Probably wan't fix this, because it's too difficult to find out how "conditional" simulations are in the DHARMa object.

[florianhartig]

See also #117