Accuracy testing in addition to performance testing

Some functions that are available in MKL are currently not defined, as they are slower than base, but "more accurate".

This should be evaluated, first by adding the various commented functions back in, and then comparing accuracy. This might need `BigFloats` computations.

This should also take into account the option to set the MKL accuracy, and evaluate impact.

The results should be added in the documentation (see #61)

Locate the commented-out MKL functions and the existing accuracy configuration. Re-enable the candidates, compare their results with base implementations using BigFloats where needed, and evaluate the effect of the MKL accuracy option. Document the findings as described in issue #61.

Vom Indexierungsmodell aus dem Issue-Text verfasst.