Implemented an expression class that makes it possible to manipulate HFSS expressions as objects in Python (closes #24).  No example code is given at this time, but functions that use it will be implemented in the near future.
Implemented an interface to the ChangeProperty() function named add_property() (closes #22), including an example.  This example also shows basic usage of the Expression object (closes #38).
Implemented interface to CreateCircle() (closes #29), including an example.  This example incorporates simple usage of the Expression object.
Fixed the set_active_editor() example to work properly (closes #39).
Implemented an interface to AssignMaterial() (closes #27) along with an example.  The example contains a known bug that will be fixed once a volume function such as CreateSphere() is implemented (see #40).
Implemented an interface to the CreateSphere() function (closes #41), along with an example.
Fixed a known issue with the assign_material() example (closes #40), where the material for a sheet object was inadvertently being set.