Changed description of Value.asNumericValue.

Other values that can be represented by a 
NumericValue in an unambiguous way should be
allowed to implement #asNumericValue().



git-svn-id: https://svn.sosy-lab.org/software/cpachecker/trunk@13054 4712c6d2-40bb-43ae-aa4b-fec3f1bdfe4c