Add spellcheck to code review checklist.

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