enable parsing of DeclareMathOperator
```
\DeclareMathOperator{\len}{length}
```
should be transformed to
````
\operatorname{lenght}
```
by the preparser (LaTeX supports \operatorname{})
Further reference on mediawiki's supportof `\operatorname{}` available here
https://en.wikipedia.org/wiki/Help:Displaying_a_formula#Standard_numerical_functions