test_stream.py: Use float64 array as test stream

Using randint as source of test stream conflicts with simple casting
when multiplying with calibration constant. And consequently tests fail.
Casting / viewing array as 'float64' fixes this.
Replacing the numpy views with true type casts.

Views do not change the underlying data and thus do not work with
custom C code that expects raw data of the correct type.
Merge pull request #500 from gauteh/use_float64_for_teststream

test_stream.py: Use float64 array as test stream