avoid division by zero in set/getArrivalTime

[YokoHono]

For small dx and dy, localBMapSizeX and localBMapSizeY can turn into 0, which subsequently leads to a division by zero in setArrivalTime and getArrivalTime.

Added a check that assigns 1 instead of 0 as the minimum localBMapSize.

[filippi]

Hi,
It is a strange case where the burning map is just one point wide for a heat flux matrix computation, ok to have a failsafe option, but maybe a warning should be issued.