make mesh_dir behavior consistent across create_meshing_tasks and create_mesh_manifest_tasks

[jasonkena]

Currently, running the example code with default arguments mesh_dir=None

• create_meshing_tasks leads to the creation of LAYER/mesh_mip_0_err_40 (ignoring the value of vol.info['mesh'])
• create_mesh_manifest_tasks on the other hand tries to read/write from LAYER/{vol.info["mesh"]}

  igneous/igneous/task_creation/mesh.py

  Line 63 in 5f9f18f

  mesh_dir=mesh_dir,

  igneous/igneous/tasks/mesh/mesh.py

  Line 576 in 5f9f18f

  mesh_dir = info['mesh']

This and some minor README.md improvements.

[jasonkena]

@william-silversmith I added another commit that solves another unrelated bug. Currently mesh = vol.mesh.get(labels)[label] always just just raises KeyError causing bounds_from_mesh to fail when this branch

igneous/igneous/task_creation/skeleton.py

Line 211 in 784033d

if (

is evaluated (in my case, when len(object_ids)<5).

[william-silversmith]

Thank you for this!!