Resolve compilation warnings
Closes #393.
Resolves compilation warnings and CMake warnings originating from Met.3D source code. These consist primarily of usages of deprecated Qt functions, which will get removed with Qt6.
Also resolved some warnings in the CMake building process.
Compilation warnings are still arising from:
- QCustomPlot compilation
- QtPropertyBrowser compilation (will get removed after property rework and refactoring obsolete classes)
- Fortran code
libcalvar.f
, no idea how to resolve those.