add quotient ring check in 'normal.lib' functions

depends on 
https://github.com/Singular/Sources/pull/574 (merge first!)
otherwise tests will fail.

Attention: The `not isQuotientRing` in `getOneVar()` will break some tests/examples,
 butconsider that getOneVar() comments clearly state that it will fail in quotient ring.

Failing Tests:

paraPlaneCurve
paraConic
adjointIdeal
mapToRatNormCurve
testParametrization
rncAntiCanonicalMap
rncItProjEven
invertBirMap