Add a lower bound of zero for TildeTau in FixConservative for zero B

[jyoo1042]

Proposed changes

Add a lower bound of zero for TildeTau in FixConservative for zero B

Upgrade instructions

Code review checklist

• The code is documented and the documentation renders correctly. Run
  make doc to generate the documentation locally into BUILD_DIR/docs/html.
  Then open index.html.
• The code follows the stylistic and code quality guidelines listed in the
  code review guide.
• The PR lists upgrade instructions and is labeled bugfix or
  new feature if appropriate.

Further comments

[nilsdeppe]

I have no additional comments to Will's.

[nilsdeppe]

LGTM, please squash once Will's looked.

[wthrowe]

Looks good. Squash.