Changing notations

[Jazzpirate]

On the latex side: a macro to set a default notation option, globally (for the whole document) or locally.

On the viewer side: we can give notations (including arbitrary value combinations for variant, lang, arity, see #3) unique URIs and annotate with those, so that MMT can parse out how a symbolic expression (i.e. MathML) came about. Conversely, \symdef/\notations can write their notations as MathML into the xhtml (in a <script>), so MMT should be able to easily substitute notations when serving a document, or even custom notations input via a latex field(?).

Question: how do we treat multiple notations for the same symbol on the MMT side? Multiple notations should be easily implementable, but what if new notations are added later on in separate modules - introduce defined constants? Keep FLaTeX notations separately in the flatex-Plugin for MMT?