Restructure/-factor import/export module

[CasperWA]

As the import/export "module" is now one big messy file, it should be "chopped up" and re-organized into a separate tool.

The same applies for the tests for import/export.

I have started to do this on a local branch and periodically pushing to my own fork.
Here is a link to the branch, going directly into the new module under aiida.tools.importexport.

This is only a suggestion for the API, and this issue should only concern itself with the API, ideally.

Please leave comments and/or suggestions.

[CasperWA]

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