Should the `simulate` method of processes return an xarray ?

[albop]

For a model, simulate(model, dr) returns an xarray with labels corresponding to variables, time index and simulation index. It would make sense for process.simulate() to also return an xarray. Among the advantages:

• no confusion about the dimensions ordering (it's never clear whether it's a N.T or a T.N array
• we could carry variable names As a side note, for consistency, 1d iid processes should return a 1.N.T array.