Add utt.assert_allclose that raise a more informative error.

This will help remove an error that appear with different seed in the tests.
small doc update
Refactoring to have BadThunkOutput reuse str_diagnostic()
rename var for clarity
Better tests error msg in Scan and should fix the buildbot failure in float32.

The numpy.allclose() use a tolerate too high for float32.
Support python object in the comparison.

This is needed in the Scan tests.
Merge pull request #1106 from nouiz/err_msg2

Add utt.assert_allclose that raise a more informative error.