FIX: (hopefully) performance issue reported by Junna at http://forum.simutrans.com/index.php?topic=13887
FIX: remove debugging message
CODE: move code with side effects out of assert() for readability
FIX: make growth slower rather than faster as bits per month increases
CHANGE: try to preserve some fractional growth at high bpm values
Merge branch 'junna-13887' of https://github.com/pipcet/simutrans-experimental.git into way-improvements
Merge branch 'james-citygrowth' of https://github.com/pipcet/simutrans-experimental.git into way-improvements