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)