Generation of a $k$-th order CBH formula (cbhexpr)

Implementing a procedure that could calculate the terms in the in the exponent of the CBH formula up to an order $k$ specified by the user. At this point it is not clear whether this routine would be strictly necessary, but it can be expected it would for high dimensional systems. A possible approach to solve this problem would be to use the Dynkin-Campbell-Baker-Hausdorff formula presented in [32].