[WIP] add hacky defmacro to latex

[lorenzhs]

This is a starting point for adding \def macros to the Latex engine. Use case:

%% DEFMACRO REFORMAT(precision=1)
%% SELECT AVG(foo/bar) as fooAvg, MEDIAN(foo/bar) as fooMedian FROM blah
\def \fooAvg {10.1}
\def \fooMedian {9.5}

It's not ready for merging, I just quickly cobbled it together without trying to really understand the codebase

[lorenzhs]

(issues include that it gobbles up too many lines if only \fooAvg is defined and I add \fooMedian. It's also completely undocumented)

[bingmann]

Nice.