Implementation for sharded multi res mesh generation with offsets

[SridharJagannathan]

Implemented for solving:
#475

[SridharJagannathan]

In principle this seems to work.
I have just made multi res meshes of the antenna lobe (left, right) in the sharded format.
I need to just solve the test error and some docs now..

[william-silversmith]

This looks great! Just one style point, would you mind changing dataoffset to data_offset? Other than that, I'm ready to merge.

[SridharJagannathan]

Done!

[william-silversmith]

Thank you so much! When you've finished your multi-res code, would there be a public place where I could take a look? Merging.

[SridharJagannathan]

@william-silversmith: The implementation is composed of two parts:

1. The mutli-res mesh generation is here: https://github.com/SridharJagannathan/pyroglancer/blob/master/pyroglancer/meshgenerator.py
   this was based on the discussions in a neuroglancer thread a while ago, which i adapted a bit.
2. The binary file generation for multi-res sharded is here:
   https://github.com/SridharJagannathan/pyroglancer/blob/master/pyroglancer/volumes.py#L146
   Hope it is helpful for you.