bugfix: with_mask should be usable when no `_mask` is defined
Merge pull request #172 from keflavich/with_mask_no__mask

bugfix: with_mask should be usable when no `_mask` is defined
update apt-get listings (fixes graphviz install issue)
add local export of meshes (.ply and .obj in addition to sketchfab)
add docs for new feature
changelog entry for PR 173