Fix module path

Fix the import paths after the migration.
The tests can now be run as 

export PYTHONPATH=$PYTHONPATH:`pwd`
tests/priors/runPriors.py
tests/linearity/runLinearityMC11.py

I haven't fixes runLinearityMC12.py because it's essentially the same as MC11; we should try to re-factor and have one test with a cmd-line option (11 or 12).
I have also moved the output plots to the corresponding subdirectories; we don't want to clutter pwd.
Please also note that the linearity test seems to give a >3sigma deviation. I don't know whether that's expected.

Davide