Remove the Bounded requirement from RealField. Replace it by methods returning Option<Self> so that they can
still be implemented by unbounded types (by returning None). Fix #25
The ComplexField trait derives from FromPrimitive again. We can actually keep this because all its methods
return Option<Self>, meaning that it could be implemented by any type.
Boundedrequirement fromRealField. Replace it by methods returningOption<Self>so that they can still be implemented by unbounded types (by returningNone). Fix #25ComplexFieldtrait derives fromFromPrimitiveagain. We can actually keep this because all its methods returnOption<Self>, meaning that it could be implemented by any type.