getBaseField -- returns the base field of a stable or unstable Grothendieck-Witt class

Given a GrothendieckWittClass or UnstableGrothendieckWittClass object beta, this method returns the base field of the Grothendieck-Witt class if the Grothendieck-Witt class is defined over a field.

This method will return the base field of the (unstable) Grothendieck-Witt class if it is defined over a finite separable extension of either the rational numbers or a finite field of characteristic not 2. The method verifies that the zero ideal of the getAlgebra of the (unstable) Grothendieck-Witt class is prime using isPrime, which only works for ideals in polynomial rings or quotients of polynomial rings over a prime field. In particular, this method may fail even when the (unstable) Grothendieck-Witt class is indeed defined over a field. In these cases, the user can apply toField to the algebra of the (unstable) Grothendieck-Witt class to obtain the base field.