Add numpy explicitly to requirements.

[mknorps]

This PR adds numpy library requirement explicitly to requirements-all.txt.

I am a FawltyDeps (dependency checker for Python projects) maintainer. I took the liberty to check peppy source code with:

fawltydeps --code peppy --deps requirements/requirements-all.txt 

and it looked great ❤️ I see you care a lot about keeping the code clean and reusable. I just found one small problem - that numpy is not declared as a dependency and used explicitly in the code.

I would like to propose the following standard for 3rd party libraries declaration:

If the 3rd party library is explicitly used in the code, it should be explicitly declared in the requirements file.

What do you think about it?

numpy is usually installed along with pandas, but if for some reason you drop pandas or its underlying dependency changes, the code may not work.

[nsheff]

thanks for pointing this out, and fawltydeps sounds useful!