Nifty7 changes

See merge request !659

Add better error message

See merge request !658

Remove gitversion interface, docs improvements and workaround pip

See merge request !657

Add pypi related files and rework documentation

See merge request !654

The reason is that the README is shown on PyPI and this information is
not really relevant for an enduser since she can just look at the
official documentation.

Use ducc's vdot if available (more accurate)

See merge request !653