Automatically rerun `PartInv()` for a strict invariance model when the input is a partial invariance model

[marklhc]

Current users need to run the analysis twice, once with a partial invariance model and then once with the strict invariance model. However, it seems common that people will use the weighted averages of the measurement parameters (e.g., tau, lambda, theta) for the strict invariance model, so I think it makes sense to add an option to PartInv() and related functions to do it automatically. I will draft a function for computing the weighted averages.

On the shiny side, it may be helpful to add a checkbox option for doing that.

[marklhc]

Need to test the merged changes from pkg-weighted-ave to pkg