Slight restructure the SVG grid output.

Slight restructure the SVG grid output.

In particular:
* Make the svg_project_point() a function in an anonymous namespace
  in the .cc file, rather than a static member function of the class.
* Make those of its arguments that represent directions into tensors,
  rather than Points.
* Move a couple of variable declarations next to their initialization.

As far as functionality is concerned, represent cells by
'level.index' rather than 'level,index', since the former is what you
get when you pass a cell to a std::ostream. Also just use 'level'
in the SVG caption, rather than 'level_number'.