Handle unit conversions on properties

[dokempf]

We currently have the situation that we allow a user to specify certain properties in degrees, but internally convert them to radians. This leads to the confusing situation that setting and getting a property directly after one another will not yield the same result:

from helios.scanner import ScannerSettings
settings = ScannerSettings()
settings.rotation_start_angle = 180
print(settings.rotation_start_angle)

This will print 3.14... instead of 180. In a CLI world this never mattered. In a Python API it is rather inconvenient.