## Rename mathematical constants such that they differ from macros in math.h

In `src/core/math/Constants.h`

, we (re-) define mathematical constants as `constexpr`

. Each constant is given the same name as in the preprocessor macros from the C-header `math.h`

(e.g. Pi is named `M_PI`

). We therefore have to `#undefine`

each constant's macro definition in `src/core/math/Constants.h`

.

However, this approach might (dependent on the order of the included header-files) fail at compile-time, once an included external library redefines an above mentioned macro.
For instance, this problem currently occurs when using the constant `walberla::math::M_PI`

while also including OpenMesh, which has `#define M_PI`

in its `MathDefs.hh`

header.

For this reason, we should rename the mathematical constants in `src/core/math/Constants.h`

such that they do not coincide with the names of macros in `math.h`

.