Constructor for sigvar object
>> w = sigvar(s)
>> w = sigvar(s,e)
s Array of signal values
If no signal, s = []
e Variances on values
If no variances, e = []
- Can be empty even if there is a non-empty signal, in which case variances are ignored
- If not empty, then must have iseqaul(size(s),size(e))==true