Use `minLevelCell` instead of an argument to determine the number
of inactive top levels.

Store the `inactive_top_comp_subdir` so we know what files to
compare with during validation.

Use `ds.load()` when modifying `minLevelCell` in the initial state
to prevent issues with reading from and writing to the same file.

Crop inactive top cells after writing out the modified initial
state (because we now use the modified `minLevelCell`).

Set the default to `inactive_top_cells = 0` in a new `init.cfg`
instead of the config file for the QU240 mesh.

Move cropping of inactive top cells to ForwardStep so it is
explicitly included as an output of that step.

Give `init.nc` as the mesh file for cropping.

For validation, point to the output file from the version of the
performance test without inactive top cells for `filename2`.