This changes a lot in the build process for the python interface. A
simple `pip install .` will now use PEP 517 and scikit-build, which
allows the installed imagefusion python package to be used by other
packages to link against. The installation directories are now

* .../lib/ for libimagefusion.so
* .../bin/ for the binary utilities
* .../lib/python*/site-packages/imagesfusion/ for the interface lib
* .../lib/cmake/**/ for the exported cmake files (also for spdlog)

This has been tested with a miniconda distribution, and with the pip3
version of the linux distribution (Ubuntu 20.04), with some dependencies
installed. In future the pip install procedure should work without
pre-installed dependencies.

However, the python interface can be installed without pip, but with the
usual `sudo make install`. The above structure will be similar, but
usually with the prefix /usr/local/ and instead of site-packages, it is
called dist-packages.