Fix subtensor merge

The formula for getting an element from an index in a reversed sequence
was wrong.

- Simplify and move test case for the bug reported by Razvan
- Add more general test of merging tensor[slice][index]

Scan revert fix

The problem was that CudaNdarrays would not allow you to zero some part of
them. My fix was to allocate a ndarray and have then the CudaNdarray setitem
to copy that memory on the card. Fred fixed the bug on the CudaNdarray side
though so this fix (which is suboptimal) is not needed anymore.

Sandbox pseudo-inverse

transpose method and tests

Fix tests sandbox scan

There are two things to wait for :
 1) Fix bugs in sandbox scan
 2) Add more restriction to scan which would simplify this test

avoiding name collision and other things

also make sure it has the right sign

initial state can be one dimension less if the taps equal just [-1]