Removing NXstage_lab after discussion with @lukaspie as a result of w…

…hich NXstage_lab will be replaced in NXem and NXapm appdefs via NXmanipulator from #1424 augmented by remaining concepts and contextualizing top-level doc string from NXstage_lab pulled into the appdef. As a side effect thereby also docstring become more focused on the relevant techniques and a base class is not talking only with examples from atom probe