require numexpr / numba

[jreback]

pandas already supports lazy evaluation via numexpr. adding numba allows the possibility of generation of lazy expressions (and recent versions support ahead-of-time compilation).

I would simply make these requirements; they are available on all platforms and just help.

xref: pandas-dev/pandas#14324
xref: https://github.com/shoyer/numbagg

xref #7

[wesm]

I'm -0 on this because these tools are NumPy-centric and do not have good support for non-numeric data.

[shoyer]

Also, Numba is still hard to install without conda. I'd like to see wheels and debian support (numexpr has both) before calling it generally "available on all platforms".