add vcov(standardized=, free.only=) arguments

[TDJorgensen]

I’m working on some lavaan.mi functionality in semTools::monteCarloCI(), which requires the full vcov() for a standardized solution. When I last visited, I worked out a hack for standardizedSolution.mi() by creating a fake lavaan object with pooled results inside it. That object can also be passed to the workhorse:

lav_object_inspect_vcov(fit, standardized = TRUE, free.only = FALSE)

But that is not a public function. I explored 2 possibilities to call it indirectly via public functions, both of which have a limitation I need to circumvent:

• Running lavInspect(fit, "vcov.std.all") only sets free.only=TRUE, without any way to pass more arguments via ... because lavInspect() only has a what= argument. It does not seem optimal to create even more what= options with long names to allow for the combination with defined/joint output.
• The vcov() method does not have a free.only= argument (type="free" only indicates whether user-defined parameters are included), nor a standardized= argument. This seems like a better candidate, because there are already some other undocumented arguments (which CRAN doesn't seem to care about, since you define the method within the setMethod() call).

This PR adds these 2 arguments to your vcov() method to pass them to lav_object_inspect_vcov(). I left these new arguments undocumented (like the existing ones) to avoid advertising them.

[TDJorgensen]

reprex to test it:

fit <- cfa("visual  =~ x1 + a*x2 + b*x3",
           constraints = 'ab := a*b',
           data = HolzingerSwineford1939)
vcov(fit) # default is unchanged, 6 x 6
vcov(fit, standardized = "std.all") # 6 by 6
vcov(fit, standardized = "std.all", type = "user") # 7 by 7
vcov(fit, standardized = "std.all", free.only = FALSE) # 8 by 8