Fixed python conservative stretching, fixed a bug in buffer allocations

...ackend/host/python/operator/directional/stretching_dir.py

hysop/core/memory/memory_request.py

hysop/deps.py

hysop/fields/cartesian_discrete_field.py

hysop/numerics/odesolvers/runge_kutta.py

hysop/numerics/stencil/stencil.py

hysop/operator/base/stretching_dir.py

hysop/operator/directional/stretching_dir.py

hysop/operator/tests/test_directional_stretching.py