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.

Fixes issue #1198