Correct the Python import statements

The addition of the path of `BA/Wrap/Python/__init__.py` to `os.path`
is _not_ necessary anymore.

The import statements need to be changed:
```
import ba_plot -> from bornagain import ba_plot
import ba_fitmonitor -> from bornagain import ba_fitmonitor
```

The root-path of the project is correctly set in the shell script
`thisbornagain.sh`.

Consult e.g. "The Definitive Guide to Python import Statements" <https://chrisyeh96.github.io/2017/08/08/definitive-guide-python-imports.html>

Tested under Debian Buster, Python 3.7.3.

Resolves issue #1198