use radius length 1
add test for conversion without rotation
rename testCoordinate
add test for conversion with rotation
rename test
add test scaling radius
refactor : rename variable to ScaleRadius
add tests for scaling down and negative scaling -> reverse direction
improve documentation of setScaleRadius
refactor : rename setMaxRadius to setScaleRadius
add test for negative angle
Coordinate center should not be null
test for null Coordinate
use constant