Reimplement the DLSCH preprocessor

This refactors the preprocessor. The new preprocessor implements the
same scheduling strategy (Modified Round-Robin: give resources according
to round-robin, then group) but should be fairer by changing every time
the RR start UE (i.e., 1st TTI UE 0 starts, 2nd TTI UE 1, etc) when the
UE number is not a divisor/multiple of the RBG size (and the last RBG is
often smaller). This also removes all unneeded code of the preprocessor
such as allocate(), positioning(), etc.