Cleanup: make import style consistent across project

Currently we use several different import styles across the project. For consistency we should agree on one and use it whenever possible. Apparently it is safe to use `from module import something`, *as long as you go to the lowest level module possible*, so I propose using that since the code looks cleaner.

```python
import ansys.common.variableinterop.real_value as real_value # good
import ansys.common.variableinterop as acvi # bad (but safe for tests/client use)

from .real_value import RealValue # good
from . import RealValue # bad
```

Start by inventorying the Python import statements across the project and review the examples in the issue. Determine whether a single lowest-level import style is appropriate for production code, tests, and client use; done means the project follows an agreed convention consistently.

Written by the indexing model from the issue text.