Improve test coverage

[rheitjoh]

It would be nice to have tests for the following methods (or investigate why there is no test coverage in case IntelliJ is wrong):

• HashIntoZnAdditiveGroup and HashIntoZp have no coverage at all (although latter should be covered by HashIntoZn)
• PolynomialRing.Polynomial#scalarProduct(Polynomial)
• ExtensionField is theoretically covered by the bilinear group tests, but should also be tested in the RingTests
• None of the new Vector type classes have any test coverage and the other cartesian stuff also doesn't
• The boolean ring structure has no test coverage
• RingGroup has no test coverage, but perhaps not necessary here
• The exception condition for GroupElement#pow(RingElement) is never hit
• For some reason none of the converter classes have coverage although they are tested in our ConverterTest.
• Boolean expression stuff has no coverage

For some reason some stuff that is definitely tested has 0% test coverage such as our representation converters, or RingElement#mul(BigInteger), Ring#extendedEuclideanAlgorithm(List<RingElement>).