additional variables/attributes to be stored in section or transport dataset

more a note than an issue...
just thinking about what else should be implemented to be extracted from the "original" data?
This is easy to select within function shift_grids by adding them into optional argument
vars_to_keep=('vozocrtx', 'vomecrty', 'votemper', 'vosaline', 'vosigma0')

I would recommend to include the following variables:

• sossheig : necessary to include in freshwater/heat computations
• somixhgt : interesting to have for quick mixed-layer depth visualization along the section
• include some attributes to the variables like 'units' and for freshwater computation also the 'reference salinity' used (this fits into the function calculate_transport)