Resolve "Add a stretching Python operator (not symbolic)"

Closes #23

See merge request !6

Resolve "Add fortran fftw Poisson operators"

Closes #24

See merge request !8

Fix hostarray nanmin and nanmax. Fix default timestep parameter.Fix Fortran FFTW…

See merge request !7

Add a python stretching (non symbolic) as a new operator StaticDirectionalStretching, copied from DirectionalStretching. These two should be merged later. Improve stretching test. Need to be fixed.

Fix hostarray nanmin and nanmax. Fix default timestep parameter.Fix Fortran FFTW diffusion operator. Add fftw diffusion test.