Test of VASP INCAR writing

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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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Implementation of a test for writing INCAR files for VASP. It tests that different types are written correctly in the INCAR.

This merge request lays some groundwork for refactoring the way INCARs are written. INCARs are currently written by write_incar in GenerateVaspInput. The method and class are very long, so extracting the logic for writing INCARs to individual functions would be highly desirable.