Add a helper function to compute explained variance (R2) values

[brandmaier]

[aaronpeikert]

The function is not exported in the PR so users will not be able to access after loading. Also testcase is missing. Also, it accesses "private" fields quite intensely (e.g. fitted.model.observed.observed_vars. The idiomatic way would be to use accessor functions. I know that Maximilian does this also quite often as well but other imply types will not have for example fitted.model.implied.F.