Installation can fail if SimpleITK is not installed prior

[nikitamoriakov]

Describe the bug
If SimpleITK is not already installed, running python setup.py install seems to force it to try to build SimpleITK from source, which can fail if permissions are insufficient. In my case, it complained about insufficient permissions and sandbox violation. However, if SimpleITK is already installed, installation of xdrt goes without issues.

To Reproduce
Create a virtual environment without SimpleITK installed. Running xdrt installer can force it to try to build and install SimpleITK from source (not expected since SimpleITK is available via pip).

Expected behavior
If SimpleITK is not available, it should try installing it via pip first, presumably.

Environment
Operating System: Mac OS