Fixed circular import issue

Sorry, I should have tested more thoroughly this stuff with GPU before
pushing it :/

Fix tolerance in Usmm test.

Added missing import

Fix when forcing cuda to be disabled

Fixes for Python 2.4

The GpuOp class must be available even when Cuda is disabled (e.g. by
forcing cuda_available = False). Otherwise some tests fail.

This commit moves the definition of GpuOp directly into cuda/__init__.py
so that it works.

Minor fixes

Update NEWS.txt

Make blas functions work with all stride patterns

Fix dtype in sparse test.

Mixed changes

Test nvidia driver

This allow to make sure the test for the driver was executed.

raise an exception if the test fail. This is because some nvidia driver version don't correctly handle our reduction code.

Both BLAS and Numpy were trying to be too clever, BLAS by wanting a
pointer to the beginning of the memory chunk (even if it will start from
the end, and Numpy by providing a pointer to the first element, that is
at the end of the memory buffer.

PyArray_GETCONTINUOUS considers 0-sized arrays as contiguous,
even if they have negative strides on the 0-length dimension for
instance, which confuses BLAS.