Consider removing `BASE_SURFACE` from `SURFACE` keyword

As far as I can tell, `BASE_SURFACE` is only used to fetch the dimensions when creating a `SurfaceConfig`.
It can lead to subtle bugs that give error messages that are difficult to understand:

`cannot reshape array of size 6 into shape (3,3)`

To get an error such as this, change `ncol` of `base_surface` to `3` in `test_surface_param_update` and change the name of the `BASE_SURFACE` file to `surf_base.irap`.
This leads to a mismatch in dimensions between base surface and the surfaces of each realization.

Perhaps we could delay adding dimensions to `SurfaceConfig` until surfaces are available and not rely on base surface at all.

Start with test_surface_param_update and reproduce the reported dimension mismatch by changing base_surface.ncol and renaming the BASE_SURFACE file. Trace how BASE_SURFACE supplies dimensions while SurfaceConfig is created. Done means the configuration no longer depends on BASE_SURFACE dimensions and the mismatch produces a clear, covered outcome.

Written by the indexing model from the issue text.