Renaming of SliceMapper to Slicewise etc.
Slicer renamed to Crop (interface introduced and implementations adapted)
Test for the slice moved to crop-package and according classes renamed.
Fixed crop tests.
Output of the crop-operation implementations are optional (and both, input and output).

If the output is not set (null) the crop operation will be performed virtually.
Tests are adopted accordingly.
MapView: tweak javadoc
MapView: remove unnecessary semicolon
WIP
WIP