Improve default test rotations
Improve default test rotations

This sets the default test rotations start and stop about the predicted
center, and it also sets the default slice number to be the middle of
this range.

Signed-off-by: Patrick Avery <743342299f279e7a8c3ff5eb40671fce3e95f13a@kitware.com>

OpenChemistry/tomviz,OpenChemistry/tomviz,OpenChemistry/tomviz,OpenChemistry/tomviz