largeop properties

[davidcarlisle]

This PR addresses issue w3c/mathml#482

It groups the more common largeop (Unicode n-ary or integral characters) in a style similar to the default fixity list. and now explicitly references largeop from the core property list.

Currently it is a separate section although an alternative more compressed layout could be achieved by simply adding largeop
to the default fixity section and dropping teh extra wording around it, relying on the link to largeop in the core property list for the speech templates.

largeop and the fixity properties are now all links to the core property list. this highlights the fact that nofix as cuuent lused eg for diameter https://davidcarlisle.github.io/mathml-docs/intent-core-concepts/#diameter has no definition.

I'm not sure if nofix was intended to be a meta-property implying there is no property applied or if it is intended to be a real property that can be explicitly used (in which case it should be added to core property list). Currently it makes a broken link.

In order to fit integrals in this largeop list I had to increase the arity so integral:largeop{f) is $\int f$ which means the arity one form can't be used for $\int_C$ perhaps we need two properties one that does not expect the summand/integrand so can be used on the munderover and just refer to the "embellished operator"?