### 2.34.4 Intervals defined by their center : `center2interval`

`center2interval` takes as argument a vector `V` of reals
and optionnally a real as second argument
(by default `V[0]-(V[1]-V[0])/2`).

`center2interval` returns a vector of intervals having
the real values of the first argument as centers, where
the value of the second argument is
the left boundary of the first interval.

Input :

`center2interval([3,5,8])`

Or (since the default value is 3-(5-3)/2=2) :

`center2interval([3,5,8],2)`

Output :

`[2..4,4..6,6..10]`

Input :

`center2interval([3,5,8],2.5)`

Output :

`[2.5..3.5,3.5..6.5,6.5..9.5]`