Storable raw event class

This MR contains a first version of a storable raw event class (CbmRawEvent), containing digis for all detector systems. The implementation is the easiest oine, with one vector per system, proper accessors and modifiers.

The event builder (CbmTaskBuildRawEvents and CbmAlgoBuildRawEvents) was modified to make use of this class. Since reconstruction need still be adapted to read from this class instead of from CbmEvent, the old classes were left untouched in order not to break the sim/reco chains. Instead, new classes were introduced (CbmEvbuildRawAlgo and CbmEvbuildRawTask). An example macro is run/macro/run_reco_mcbm.C (only event building, no reconstruction).