Simulations to check permutation type I

[behinger]

This is my first suggestion:

minimal

- y ~ 1 + (1|subject)
- 

basic

- y ~ 1 + cat*cont + (cat*cont||subject)

ranefs

- y ~ 1 + cat*cont + (cat*cont||subject)+ (1+cat || item)

correlated ranefs

- y ~ 1 + cat*cont + (cat*cont|subject)

I have scripts in matlab to simulate this exact design, but I'd rather use https://github.com/RePsychLing/MixedModelsSim.jl and reimplemente it, to be more flexible in design.