euler_phi_inv for 1

[SoongNoonien]

According to the docstring I'd expect euler_phi_inv(1) to return a vector containing both 1 and 2:

The inverse of the Euler totient functions: find all x s.th. phi(x) = n holds.

But currently it just returns a vector containing 1 as its only element.

[fingolfin]

So either the docstring should be adjusted, or the behavior for this input.

Any preferences @fieker @thofma ?

[fieker]

the behaviour