map origin unit issue

[mildsky]

hector_slam/hector_mapping/src/HectorMappingRos.cpp

Line 547 in b7d19bf

mapOrigin.array() -= gridMap.getCellLength()*0.5f;

if you check this line, it uses gridMap.getCellLength() function that returns cell length in millimeter.
but, according to message definition, it should be meter, not millimeter.
So I think it should be corrected as below

    mapOrigin.array() -= gridMap.getCellLength()/1000*0.5f;

I created pull request, so check it please.