add test of array input to SnapshotPotential (related to #211)
rm unnecessary imports
fix array R,z behavior, use interpRZPotential decorator, fixes #211
rm unnecessary import
test of force as derivative of the potential for SnapshotPotential, related to #215
add a test of InterpSnapshotPotential to test_potential, for now fixes 215 (no orbit tests make sense now)