tests: XML round-trip test suite and three serialization bug fixes

Add test_XML_RoundTrip.py covering all property and primitive types via
Write2XML/ReadFromXML with non-default values. Three bugs found and fixed:

- CSPropProbeBox: Weight/StartTime/StopTime used SetAttribute (int) instead
  of SetDoubleAttribute, truncating fractional values.
- CSProperties: bVisisble was never serialized; SetVisibility(false) was
  silently lost on re-import.
- CSProperties.pyx: CSPropLorentzMaterial weight functions passed Python str
  to C++ without .encode('UTF-8'), unlike the equivalent Debye code.

Signed-off-by: Thorsten Liebig <thorsten.liebig@gmx.de>
Generated-by: Claude Sonnet 4.6

Author: thliebig

python/CSXCAD/CSProperties.pyx

@@ -1503,6 +1503,7 @@ cdef class CSPropLorentzMaterial(CSPropDispersiveMaterial):
             CSPropDispersiveMaterial._GetDispersiveMaterialWeightDir(self, prop_name, order, ny)
 
     def _SetDispersiveMaterialWeightDir(self, prop_name, order, ny, val):
+        val = val.encode('UTF-8')
         if prop_name == 'eps_plasma':
             return (<_CSPropLorentzMaterial*>self.thisptr).SetEpsPlasmaFreqWeightFunction(order, val, ny)
         elif prop_name == 'eps_pole_freq':