Add file managing methods to CST_MicrowaveStudio class

Methods to be added:

openFile(folder, filename):

Open the project filename in the specified folder. If the project does not exist, create a new project.

In both cases, store folder and filename to class attributes (hidden) for later saving the file.

After opening the project, return the COM object referencing the MWS.

saveFile(folder="",filename=""):

If both a folder and a filename are passed, check that extension of the file is ".cst" and update the folder and filename attributes with the new values.

Then, save the CST project at the location indicated by the attribute folder and with the name indicated by the attribute filename.

If only a folder or a filename is passed, raise an exception.

closeFile():

Closes the current active project.

quit():

Closes CST application