Set Model attributes only if needed

[azimgivron]

Description

I made a minor adjustment to the model constructor: model elements are now initialized only if the corresponding argument is None. This change was made to support the following use case.

class MyGeometry(openmc.Geometry):
        def __init__(self, my_attr, **kwargs):
              self.my_attr = my_attr
              super().__init__(**kwargs)

class MyModel(openmc.Model):
        def __init__(self, 
            geometry: MyGeometry = None,
            materials: openmc.Materials = None,
            settings: openmc.Settings = None,
            tallies: openmc.Tallies = None,
            plots: openmc.Plots = None
         ):
              super().__init__(
                     geometry,
                      materials,
                      settings,
                      tallies,
                      plots,
              )
        
          @openmc.Model.geometry.setter
          def geometry(self, geometry: MyGeometry):
              """Geometry setter.
      
              Args:
                  geometry (MyGeometry): The geometry.
      
              """
              if not isinstance(geometry, MyGeometry):
                  raise ValueError("The geometry must be of type {MyGeometry.__name__}.")
              openmc.Model.geometry.fset(self, geometry)

Without the change, setting the geometry to an openmc.Geometry will raise an error.

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 15) on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)