Unsigned Integers

[ax3l]

Fortran does not support unsigned integers without extensions.

Although we can just store/save openPMD (HDF5/ADIOS2/etc.) variables to signed integers in Fortran, because the signed but is the most leftmost on architectures used today, this will result in the same values for all but the values close to the upper value range - this can be simplified.

Let's check where we require unsigned values and see if we can also use signed value types there.

cc @tsung1029