Fix broken TofConverter gui.

Refs #10162.
Fix tabs and other issues in convertGUI.py

Other issues fixed:
- pi was hardcoded and never used
- e2cm had been mistyped
- blank line usage was inconsistent

Refs #10162.
Fix error when converting to momentum transfer.

The gui would complain that theta needs to be > 0 when a value had been
entered correctly.

Refs #10162.
Merge remote-tracking branch 'origin/bugfix/10162_tofconverter_gui_is_broken'