Update vasp_to_xml.py

[knc6]

Fix https://github.com/usnistgov/jarvis/blob/master/jarvis/db/vasp_to_xml.py

• [x] Make sure vasprun.xml is converged, especially for TBmBJ
• [x] Make sure negative bandgaps are reduced to zero especially for TBmBJ, e.g. https://www.ctcms.nist.gov/~knc6/static/JARVIS-DFT/JVASP-42538.xml
• [x] Make sure the label is spin-down in SOC spillage, not trace (https://www.ctcms.nist.gov/~knc6/static/JARVIS-DFT/JVASP-8122.xml)
• [x] Fix order of elements in formula, e.g. https://www.ctcms.nist.gov/~knc6/static/JARVIS-DFT/JVASP-2817.xml
• [x] Make sure See also links exist e.g. https://www.ctcms.nist.gov/~knc6/static/JARVIS-DFT/JVASP-101901.xml
• [x] Epsilon in ir spectra
• [ ] Check bandgap