Installable PyFANS

[sanathkeshav]

• remove fetch content and make pybind11 a dependency
• PyFANS is now installable as import PyFANS
• minor changes to the test

Checklist:

• I made sure that the CI passed before I ask for a review.
• I added a summary of the changes (compared to the last release) in the CHANGELOG.md.
• If necessary, I made changes to the documentation and/or added new content.
• I will remember to squash-and-merge, providing a useful summary of the changes of this PR.