Fix problem with vector handling in ST3 derivatives

[BerriJ]

The ST3 derivatives enforce various bounds on the dsq1 and dsq2 objects using ifelse.
However, ifelse poorly handles non-matching lengths of the inputs. Consider this example:

ifelse(TRUE, c(1, 2), c(3, 4))

This evaluates to 1.

Similarly in ST3()$dldm

w1 <- ifelse(tau < 1000000, (tau+1)/(tau+dsq1),1)
w2 <- ifelse(tau < 1000000, (tau+1)/(tau+dsq2),1)

Evaluate to the first element of the vector (tau+1)/(tau+dsq1) and (tau+1)/(tau+dsq2). I believe outputting the whole vectors (tau+1)/(tau+dsq1) and (tau+1)/(tau+dsq2) would be correct here.
I suggest to fix that by repeating the input so that it matches the length of y which determines the length of dsq1 and dsq2. Like so: rep(tau, length(y)).

This is what this PR does.

Best regards,
BerriJ

[BerriJ]

Did you have a chance to review this?
Since this is a non-technical pull request and gamlss depends on it, it would be nice to get this merged.