Radix sort: handle more data types

Signed integers are currently mishandled:

>>> arr = cl.array.to_device(queue, np.array([-1, -2, 0], dtype=np.int32))
>>> RadixSort(ctx, ["int *arr"], "arr[i]", ["arr"], key_dtype=np.int32)(arr)
([array([ 0, -2, -1], dtype=int32)], <pyopencl._cl.Event object at 0x12e2da1a8>)

IEEE floating point could also be handled.

changed title from Radix sort doesn't handle negative numbers to Radix sort mishandles negative numbers

changed the description

changed title from Radix sort mishandles negative numbers to Radix sort: handle more data types

changed the description

Thanks for the report! It occurs to me that IEEE floating point can also be handled through radix sort. We just need to make sure the driver code recognizes that.