NaN validation, rename provate vincentyFormula, JavaDoc tweaks

- Clarify some documentation
- Update JavaDocs to avoid warnings on recent JDKs
- Rename the private vincentyFormula to vincentyInverseFormula. There is a direct formula as well that I have implemented but not added since it is of little value for zmanim
- Add NaN validation
- Change some spaces to tabs for consistency

Author: KosherJava