Changed BasisCoeffcientsOfRingElement to avoid needless computation.

BasisCoefficientsOfRingElement returned the first row of the representation matrix computed by RepresentationOverCoefficientsRing.
Now I compute this vector like in RepresentationOverCoeffcientsRing and do not compute the remaining rows which are not needed.
Fixed a mistake in CoefficientsTransformation and fixed the corresponding examples.
Fixed the example AppendToGroebnerbasisOfZeroDimensionalIdeal.g
added some comments.