Macros, latexify, snapshot

Introduction of shapshot enables watching the sketch develop, and also enables animation and zooming without redrawing. It uses Cairo's recording surfaces.

The macros @eq_draw and @ev_draw enables rendering expressions while simultaneously rendering as latex.
The rendering for this package is done with NodeJS, but small adaptions enables translation to Katex for html output, too.

The tests don't fully keep up with the changes. We realized we ought to move some functionality out to MechGluecode.jl, and we still don't know the best home for some functionality.