add option allow.empty.cell

[ecmerkle]

This adds an extra option allow.empty.cell, which allows for ordinal variables that are missing some categories. This could involve an empty category in one group of a multi-group model, or a variable of class factor that (say) has 4 levels but only 3 are observed. My goal here is to have lavaan process these ordinal variables, so I can then model them in blavaan (where priors could compensate for the missing categories).

The default is allow.empty.cell = FALSE, where everything should work as usual.

For allow.empty.cell = TRUE, I had to set arbitrary starting values for the thresholds around empty categories. Assuming a variable with K categories, here is what I did:

Empty category 1: lower threshold -Inf, upper threshold -4
Empty category K: lower threshold 4, upper threshold Inf
Empty other category: upper threshold = .01 + lower threshold

[ecmerkle]

will fix these failed checks...

[yrosseel]

Nice. Thanks! Merged now.