rm remnant from debugging
rm LabeledDiGraph.default_export_path, .default_export_fname

- simply pass the path as part of the filename
- set LabeledDiGraph.name, default_export_fname is redundant
simplify LabeledDiGraph.save

for details read its new docstring
new: option to save dot for latex

- prepends numbers following letters with underscores, so 's0' -> 's_0'
- encloses states, edges and their labels in $ (math mode)
- escapes {, } with \, to make them visible after typesetting