[WIP] Add repo provider for MECA Bundles

[stevejpurves]

This PR adds a content provider for MECA Bundles.

MECA stands for Manuscript Exchange Common Approach, a NISO standard for transfer of manuscripts between manuscript systems.

This RepoProvider requires a parallel ContentProvider to be in place in repo2docker -- see this repo2docker PR for more background and details

TODO

• pytest tests - question? best place to host a "test" meca file, in this repository?

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[sgaist]

The implementation looks sane. There are only some minor things to fix/update.

[sgaist]

For the test MECA bundle, I think that the tests folder already contains mock data used for the tests. See the various .json files the folder contains.

[sgaist]

Following the test error, either name should be only MECA or launch.json changed to contain MECA Bundle.