Pcg-skel

Added

* `pcg_meshwork` takes the same `save_to_cache` argument as `pcg_skeleton`.

Changed

* Voxel resolution is always inferred from the mip-0 resolution of the segmentation cloudvolume.
Note that this means that if the segmentation mip-0 resolution is not the same as the normal working resolution, `root_point_resolution` must be explicitly set.

* The package explicitly uses the authentication token set in the client to instantiate cloudvolume objects, enabling it to work with non-default tokens.

* `pcg_meshwork` now sets the point column for synapses for better integration with meshwork features.

Fixed

* Fixed bug in `get_level2_synapses` prevented `post=True`, `pre=False` situation from working correctly.

* Improved default parameters in call cases where a CloudVolume instance is initialized.

Changed

* Choose your mip level when falling back to segmentation.
This saves a bit of time on the download, and if the level-2 object has been downsampled out of existence at this mip level, it will rerun at mip-0 where it must exist.

* More aggressive handling of segmentation memory use. Requires cloudvolume renumber mode.

Added

* *Localizing level 2 ids via segmentation if the mesh does not exist.*
There are both expected and unexpected an unexpected reasons for mesh fragments not to exist.
Unfortunately, this situation seems to crop up for virtually every neuron.
However, all level 2 ids must be associated with at least one supervoxel, so we can use the segmentation itself to do the same operation.
This is a bit slower, since we have to download the whole chunk, but it's a robust fallback that can be used when no mesh is found.
This is now used by default, but can be turned off with `segmentation_fallback=False` on any of the functions that use level 2 positions.

* *Improved parallelization.*
To speed up the slow segmentation fallback, parallelization occurs in more processing steps.

Initial release