makes it easier to re-use elsewhere

The test_Interpolate unit test has been reported to fail when executed
on different platforms (arm64 in this case).

The test_simulation_energy_calculations unit test compares the potential
energy of crambin after 200 steps steepest descent with a reference value.
Depending on OpenMM version / hardware, we might get slight differences
that propagate through the minimization.

CMake now takes care of extracting the major, minor and patch version of Python

I assume there was good reason to add it...

You might have to adapt CMake flags when building OpenStructure to variables specified here:
https://cmake.org/cmake/help/latest/module/FindPython.html

First entry is about adding the -pthread flag to resolve linking
problems. Has been observed on the scicore cluster and reported
by Yuri (FreeBSD 12.2).

The previously used URLS were redirected to the new ones but redirection
is not guaranteed to last indefinitely.
related documentation: https://www.rcsb.org/pages/download/http

Unit tests successfully run through with Python 3.9 on Fedora 33 (no GUI)

This caused trouble when switching OpenGL context for offscreen
rendering (DNG image export)

- Keep track of the membrane facing atoms that contributed to the energy
  function and map their energies as float properties to the input entity
- Don't consider atoms with energy contributions of 0.0 in the optimizer to
  to speed things up

When exporting the scene for the second time, weird artefacts that looked
like shadows occured. The fix enforces to re-allocate the resources of
the offscreen QOpenGLContext. Measurements showed that reallocation is
much faster than the rate-limiting step of the Export function: dump the
file to disk. Performance impact is therefore limited. Feel free to
investigate and propose a more elegant fix...