oocgcm does not trivially apply to mitgcm outputs

[serazing]

I have worked on adding the grid of the mitgcm, it requires a few hack to make it work with the current API:

• I do not have a mask with the mitgcm files so I used the 'cell_vertical_fraction_at_t_location' instead. It seems to work for the moment even if it is not the best solution
• I skipped the chunk checking
• I have relaxed the condition on the required array and have added a function to generate the the cell_size if needed

I have successfully computed some gradients but the laplacian does not work. I think there is some work to do to test if oocgcm apply well to different models.

I have pushed a branch to keep working on it, @rabernat you might be interested in.