[cleanup] Canonicalize SFINAE in referenceElements()

The used version of SFINAE/enable_if has the advantage of not adding additional template-parameters. However, it has the disadvantage of making the generated doxygen documentation hard to read.

To improve the situation, this commit changes this to SFINAE/enable_if by defaulted int template parameter. This additional parameter also shows up in doxygen but it's much easier to identify as SFINAE dummy because it follows a simple idiomatic pattern. More importantly it does not mess up the documentation for the other parameters.