Better error message if problem in givens

Fix max'd error in EquilibriumOptimizer with constant folding

Now constant folding does not bust it anymore.

It used to wait for the first attempt to apply an optimization
after it has already crossed the threshold.
Now, it signals that the optimizer should stop earlier, just after
crossing that threshold.

Instead of using current size of the graph, use the size the graph had
at its largest. This enables optimizations like constant-folding to
reduce greatly the total number of nodes in the graph without triggering
an error.

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.