Memory management improve + methods in operators. Almost all tests ok

HySoP/hysop/methods.py

HySoP/hysop/numerics/differential_operations.py

HySoP/hysop/numerics/tests/test_diffOp.py

HySoP/hysop/numerics/tests/test_integrators.py

HySoP/hysop/operator/discrete/penalization.py

HySoP/hysop/operator/discrete/poisson_fft.py

HySoP/hysop/operator/discrete/stretching.py

HySoP/hysop/operator/penalization.py

HySoP/hysop/operator/tests/test_Stretching.py

HySoP/hysop/operator/tests/test_penalization.py

HySoP/hysop/operator/tests/test_poisson.py

HySoP/hysop/operator/tests/test_synchronizeGhosts.py