Fix #631

python setup.py develop
followed by
mvs_tool -i tests/inputs/ -ext csv -f -pdf
should work now

previously only worked via normal pip install or python setup.py install

Changes proposed in this pull request:

• If assets folder is not found in package look in current folder for report/assets folder

The following steps were realized, as well (if applies):

• Use in-line comments to explain your code
• Write docstrings to your code (example docstring)
• For new functionalities: Explain in readthedocs
• Write test(s) for your new patch of code (pytests, assertion debug messages)
• Update the CHANGELOG.md
• Apply black (black . --exclude docs/)
• Check if benchmark tests pass locally (EXECUTE_TESTS_ON=master pytest)

_{For more information on how to contribute check the CONTRIBUTING.md.}