This is a helper to generate a formula for a bamlss model with k-dimensional
multi-variate normal distribution and Cholesky decomposed variance-covariance matrix.
It is helpful if one formula should be used for means, another for all diagonal
entries of the Cholesky factor, and a third one for all lower triangular entries
of the Cholesky factor.
The left hand side has k elements separated by |.
The right hand side has one to three elements separated by | specifying
the formulas used for all means, diagonal entries of the Cholesky factor and
lower triangular entries of the Cholesky factor, respectively.