Mathml parser
This implements a parser for the MathML spec https://www.w3.org/TR/2003/REC-MathML2-20031021/chapter4.html At the moment only content markup is implemented
Usage example
use parse_document;
let test = "<math xmlns=\"http://www.w3.org/1998/Math/MathML\">
<apply>
<plus/>
<ci> x </ci>
<ci> y </ci>
</apply></math>";
let res = parse_document.unwrap;
let exp = Root;
assert_eq!;