Allow x=c(x1,x2,x3,x4) instead of x1,x2,x3,x4 in constructors

e.g.

```
FuzzyNumber <- function(a1, a2, a3, a4,
lower=function(a) rep(NA_real_, length(a)),
upper=function(a) rep(NA_real_, length(a)),
left=function(x) rep(NA_real_, length(x)),
right=function(x) rep(NA_real_, length(x)))
{
new("FuzzyNumber", a1=a1, a2=a2, a3=a3, a4=a4,
lower=lower, upper=upper, left=left, right=right)
}
```

detect if `a1` is of length 4 and a2,a3,a4 are missing