Complex compound time signatures

This snippets implements complex compound time signatures (e.g. (1+3)/8 or (1+3)/8 + 2/4). The function \compoundMeter (defined in the snippet) can be used instead of \time to create those time signatures. The argument is a Scheme list of lists. Each list describes one fraction, with the last entry being the denominator, while the first entries describe the summands in the enumerator. If the time signature consists of just one fraction, the list can be given directly, i.e. not as a list containing a single list. For example, a time signature of (3+1)/8 + 2/4 would be created as \compoundMeter #'((3 1 8) (2 4)), and a time signature of (3+2)/8 as \compoundMeter #'((3 2 8)) or shorter \compoundMeter #'(3 2 8). The beat structure (and thus the automatic beaming) will be derived from the given meter automatically.