minor tweaks in log messages.
fixes in alembic mesh file reader.

fixed assignment of vertex normals and UV coordinates to face vertices.
assign a material index (0) to all the triangles (per-face materials are not yet supported).
removed unused directory from sandbox.
renamed the normalizeprojectfile tool to updateprojectfile.
implemented project format migrations.

the updateprojectfile tool can now be used to update a project file to the latest revision of the project format.

in a follow up commit, project format migrations will be transparently applied by appleseed.studio and appleseed.cli.