Add speed/time/distance conversion routines to simunits.h
Small comment improvement
Add passengers discarded due to waiting too long to the "too slow" graph, not the "unhappy" graph (which represents overcrowding)
Make get_unhappy_percentage readable.
Relocate main revenue calculation and speedbonus code to ware_besch_t.  Eliminate multiple bugs in goods frame by using same subroutine for it and revenue.  Reorganize early revenue code in simconvoi.cc.  Reorganize speed bonus handling and fix treatment of wtype and welt variables in gui/goods_frame_t and gui/goods_stats_t.  Fix bad uses of "const" in simconvoi.