Custodian

See changes at https://materialsproject.github.io/custodian

1. Custodian is now Python 3 compatible and uses the latest versions of
pymatgen and monty.

1. **Major** Custodian now exits with RuntimeError when max_errors or
unrecoverable_error is encountered.
2. Added BadVasprunXMLHandler.

1. auto_npar option in VaspJob now properly handles Hessian calculations.
2. WalltimeHandler now supports termination at electronic step (David
Waroquiers).
3. Improved handling of BRMIX fixes.

1. Improved backwards compatibility for WallTimeHandler.
2. Improvements to VaspErrorHandler. No longer catches spurious BRMIX error
messages when NELECT is specified in INCAR, and pricel and rot_mat errors
are now fixed with symmetry precision and gamma centered KPOINTS instead.
3. Improved Qchem error handler (Xiaohui Qu).