Explained variance (R2)

[brandmaier]

Hi,

I created a function to compute explained variance ($R^2$) values for observed variables because I want to use this in our tutorial. This should just be a standard helper function in the package. What do you think?

Here is my PR: #284

I checked the result against one exemplary model against the lavaan output and it was identical.

Best,
Andreas

[brandmaier]

At first, I called this pseudo $R^2$, which is not a good naming because this is usually used for $R^2$-type estimates in logistic models. I propose to just call the function explained_variance().