Inconsistency for isuniversal

[mforets]

In #1799 we added isuniversal. If the keyword argument witness is active, the function looks at the values, otherwise it doesn't. This leads to the inconsistency isuniversal(HPolytope()) = false and isuniversal(HPolytope(), true) = (true, N[]).

We can add a function isuniversaltype to use only the type (no witness is provided) and isuniversal which uses the value (and a witness is optional).