moved ThisInTermsOfParentSumPrimitiveElement from Term to LinearlyIndependentSum
moved ThisInTermsOfParentSumPrimitiveElement from Term to LinearlyIndependentSum

It's no longer necessary to worry about whether a Term might end up having multiple parent sums, which allowed for eliminating Term.Copy.
AlexanderKindel/solver,AlexanderKindel/solver