Fix for bug #21605, the failure of the Frame_order.test_generate_rotor2_distribution system test.
The bug at http://gna.org/bugs/?21605 is due to the fact that numpy.float16 is not defined on all
systems. Older numpy versions do not have this. Therefore the float16 value is now imported from
lib.check_types where it is aliased to float32 when not defined.
