Add or drop nodes from a semPlotModel object

[sfcheung]

This should be done before using semPaths. Functions have been drafted and being tested in the experimental branch experimental_sf.

Done in the nodes_keep_drop branch. Tested. (1e1f80f, 1984249)

[marklhc]

Not sure whether it's relevant, but semPlot::semPaths() accepts the argument structural = TRUE which only shows the structural model and drops the measurement indicators. I tested that with a few functions in semptools and I think they work fine.

[sfcheung]

Thanks, @marklhc. I added this because of a real case. We have a lot of control variables which we want to omit from a diagram (they will be included in the table of estimates). Therefore, that function does not work for us.

I already drafted them in an experimental branch (experimental_sf), intended for testing (and using) those functions in that project. You can take a look.

[sfcheung]

With those two functions, users can drop or keep any nodes they want (in principle). This can be quite useful when there are a lot of control variables, or when users when to present a subset of a model for readability.