allow merge_section to merge child classes too

[andreaa93]

When we want to import a recipe into its correspondent activity, or some king of template archive into another, merge_Section is raising an error if the sub_section_def are not the very same.

E.g. SubstrateHolderPosition and FilledSubstrateHolderPosition and so on

I sketched a solution that I will propose in a MR connected to this issue @ka-sarthak @hampusnasstrom @budschi