`collect_matrix_terms` does not handle nested `MatrixTerm`

[kleinschmidt]

https://github.com/beacon-biosignals/Effects.jl/pull/23/

This comes up when you've already called collect_matrix_terms on a collection of terms that has a mix of matrix- and non-matrix-terms (e.g., a MixedEffects.jl RanefTerm), and then call collect_matrix_terms again on the result.