Work on tests and fixes for Field

 * fixed rotate with reshape=True
 * 2d rotation: fix sign of x component
 * clip: negative values are no longer cut off for scalars
 * re-organized tests and fixtures
 * improve coverage: clip, __getitem__, rotate: scalars, comparisons, ...

Co-authored-by:  <d.weber@fz-juelich.de>
Co-authored-by:  <j.caron@fz-juelich.de>