The general form of the random effects specification is

(params1|groups1)+(params2|groups2) + ...

  where "params1", "params2" are valid fixed effect specifications
and "groups1", "groups2" are valid grouping specifications (i.e.
they expand to one or more factors

We need to be careful about terminology.

process_randformula expands this into a list:

$mmats contains the model matrices corresponding to the LHS of each term.
$codes contains matrices corresponding to the indices of each grouping factor
       in each term
$levels contains the levels of all the factors involved in all of the chunks
        (flattened)
$nterms contains the number of terms involved in the grouping factor of each chunk

The code I had worked OK for simple RHS, i.e. cases where the RHS evaluated to a single factor.  process_randformula

m: number of columns of LHS model matrix
