Add spellcheck to code review checklist.

Addresses #396.

   - [ ] Is it mergable
   - [ ] Did it pass the tests?
   - [ ] If it introduces new functionality in scripts/ is it tested?
     Check for code coverage.
   - [ ] Is it well formatted? Look at `pep8`/`pylint`, `cppcheck`, and
     `make doc` output. Use `autopep8` and `astyle -A10` if needed. 
   - [ ] Is it documented in the Changelog?
   - [ ] Was spellcheck run on the source code and documentation after changes were made?