Return individual surfaces for each coefficient in FieldGenerator/slices

[marcelbernet]

The current outputs of .slices() are the resulting surfaces of the sum of all coefficients. We discussed today that it would be good to have the option to extract the individual slices corresponding to each coefficient.

With this feature, one would avoid rerunning the FieldGenerator each time a set of coefficients is masked, and instead just sum the corresponding stored slices.

This would decrease the computing cost but increase the storage cost.