Minor fixes and new flux feature:

See merge request !78

- Fix on spatial_join.py
- Fix on cell_area return 2D array
- Flux conservative interpolation flag

Spatial join & stderr.flush()

See merge request !77

- Improved spatial_join

Bugfixing spin-up.

See merge request !76

Write by time step done

See merge request !74

- Reverse renaming of sel_time copy argument

Fix arguments in gather for points datasets

See merge request !73

Avoiding Numpy MPI gather if KeyError is raised

See merge request !72

Develop 55 concatenation improve time

See merge request !71

Develop 48 sum of variables

See merge request !70