Add Numpy to requirements_dev.txt to support Numpy in Tox virtualenvs - Tox installs dependencies in each Python version's virtualenv   using requirements_dev.txt. - No one would be able to work on the package without Numpy now anyway.
Add Numpy to requirements_dev.txt to support Numpy in Tox virtualenvs
- Tox installs dependencies in each Python version's virtualenv
  using requirements_dev.txt.
- No one would be able to work on the package without Numpy now anyway.

nsh87/ehrcorral