Update the AdvectionFieldFinder

[PaulineVidal]

Update the AdvectionFieldFinder to make it compatible with MultipatchField instead of Field.

Remark: We probably also need to be capable to deal with mixed cases (= a global Poisson solver and a multipatch advection OR with a Poisson solver not defined on the same grids as the advection solver). In this case, as the AdvectionFieldFinder computes the advection field from the electrostatic potential, we need to split the Fields into MultipatchFields OR interpolate the functions on the missing interpolation points (the points on the advection grid but not the Poisson grid). It can be done in several MR.

Remark bis: This AdvectionFieldFinder is specific to the guiding center equations. The advection field expression may differs for the axisymetric equations or other geometries.