update CI, read specific vars in npzread

[jishnub]

Firstly this PR adds the option to read specific variables in npzread instead of all the variables.

Secondly it adds docstrings to npzread and npzwrite. It also adds a documentation generation workflow.

Thirdly it updates the CI to github actions from travis, and updates Tagbot as suggested here. Tbh I'm not totally sure if this is done correctly.

[jishnub]

The readme badge needs to be updated, perhaps this can be done after merging.

[jishnub]

yes I realize now that separate PRs would have been a better idea.

The documentation workflow pushes commits to the gh-pages branch. It might be necessary to select this branch as the source of github pages from settings. Previously I've only used travis for this, and there might be issues with using github actions for this. Reading through the issue it seems that using an SSH deploy key should get around this, so I'm adding this for the time being.

[codecov-io]

Codecov Report

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