Use the PyAnsys namespace `ansys.stk.core`

[jorgepiloto]

🐞 Problem
At the moment, the namespace being followed by the API uses the old agi.stk12 namespace. As per our packaging guidelines, the namespace should look like ansys.stk.core.

💡 Solution
Most of the sub-packages and modules are using relative imports. This simplifies most of the work. We should try to use these type of imports everywhere we can.