hasn't been used/tested for years

The reason is that we want to profit from latest security updates and
therefore not use a copy of the SQLite3 source code that becomes outdated.

ToDo: test on different Linux distributions / bc2 and adapt compile
instructions accordingly, also container recipes must be adapted.

What is NOT done yet:
- OpenGL context of QOpenGLWindow must be made
  active before calling any GL function in scene.cc. The wrong
  GL context is active otherwise.
- cmake polishing
- final grepping for QT4 leftovers

changes:

-GetPrincipalAxes in Vec3List
 Results with Eigen3 have been verified with 10 randomly picked pdb structures.
 Differences only occur with changing signs of the vectors.

-CalcPrincipalAxis in BoundingBox
 No difference in algorithm, the SelfAdjointEigenSolver no lives in
 Eigenvalues. Does in principal the same thing as the function above,
 just using different functionality.

-SVD superposer
 Unit tests still run through with the changes introduced for Eigen3
 and also manual testing seems to work well.

-CartoonRenderer
 Principal axes calculation for rendering helices as cylinders had to be
 adapted. Rendering looks exactly the same...

-SuperposeFrames
 Not yet tested...

ToDo:
numeric.h and levenberg_marquardt.h in the img/alg module have to be
ported. This will be done in the next few days.

OpenMM library

run cmake with -DENABLE_MM=1
if you can successfully compile, check out the examples in the
modules/mol/mm/examples directory

fixed lddt compilation on Windows
removed DLLEXPORTS from pdbsize.cc

Refactored FindEigen.cmake to adhere to cmake guidelines
Adapted master CMakeLists.txt to changes in find modules

Substitutes DEBIAN_STYLE_LIBEXEC flag and manages location and name of lib and libexec folders on Debian/Ubuntu-based systems

fix master CMakeLists.txt to only add -Wno-attributes for newer compilers (not all older compilers have that option)