Simplify input file for DIMENSIONS < 3 setups

[glesur]

Historically, Idefix has always required the user to specify the Grid and the boundary conditions in the 3 spatial directions regardless of the DIMENSIONS parameters. This has led to confusions a number of times for newcomers, since this is actually never used in practice.

This PR attempts at fixing this: we no longer require the Grid and Boundary conditions to be specified in the directions >= DIMENSIONS.

Tests with DIMENSIONS<3 have been adjusted accordingly.