Improved testing of preallocated outputs in DebugMode

Fix a Scan test. It is normal both scan got merged.

This reflects the fact that the C linker is not necessarily the problem,
and that this exception is also raised when different calls to the same
thunk (for instance, with different preallocated outputs) give different
results.

Also, do not return a preallocated map it if is empty,
and fill "previous" storage with default value.

Otherwise, we have to allocate 2**ndims times the amount of memory,
and 2**4 seems enough.

Also, strides 1 and -1 will not be tested independently on the
dimensions above 4, their strides will always be all 1 or all -1.

The += syntax is not implemented for dims > 5.
Also actually use a tensor of wrong size in "wrong_size" mode.

If an input is viewed, and the corresponding output is destroyed,
then it destroys the input.
There would be ways to be more precise, but at least this is correct.

Fix test

Fix scan debugmode

Fix scan debugmode

destroy_map is not always defined for a scan op.

Fix tests

Looks ok to me.

Pycuda init

This is probably due to wrong handling of context. I think theano and
pycuda don't use the same context while the should. I am investigating
the problem, but while this make the buildbot segfault, I disable it
for now.

Update to sparse sandbox