exciting/bandstructure

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• I have read the contribution guidelines.
• Unit tests have been added for any new or changed code, and the test suite passes. Note: Your request will likely not be merged without the appropriate tests.
• New features and API Changes are described in doc/releasenotes.rst.
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excitingtools is able to create an ASE bandpath from an ASE atoms object and create the relevant exciting xml key-pairs to put into the input.xml file under properties. We use this functionality and modify the exciting io and calculator files to allow the user to run a bandstructure calculation. The bandstructure calculation can be performed with a ground state calculation or alone (alone if do is set to skip in the ground state properties). Tests verify that the input.xml has the required properties.

In the future, we should modify exciting tools to accept a bandpath directly since I think other codes will do the same and it makes sense to use a consistent API. It would also allow users to create custom bandpaths and not only the automatically generated ones created by cell.bandpath() which is done currently and where only the number of steps can be chosen. That said I think the current case covers 99% of users use cases.