dmm: preprocessor: multiply RHS of equations by -1 to get the right sign in the derivatives
dmm: change nile.mod test file to accord with ab59588733c4a929b55dbcb2d7dedc99b2b42157
dmm: if possible, combine two latent vars that impact the same matrix
dmm: change params to match those in the 3 eq nile example in the dmm submodule