Get financial functions out of main namespace

We have these financial functions that are exported in the main numpy namespace, and are really out of place: http://docs.scipy.org/doc/numpy/reference/routines.financial.html

Also:

• They have terrible names (even if they weren’t in the main namespace, but especially so given that they are)
• There are multiple documented, but unimplemented, functions here. WTF.

Let’s move them to numpy.financial (currently they’re also in numpy.lib.financial, but the .lib. part is just noise IMO) with better names and deprecate the current names.

[WarrenWeckesser]

The financial functions are now officially deprecated, and the plan is to remove them from numpy 1.20. We can close this issue.

[seberg]

The plan is to create the new pypi module before we put in a deprecation warning, and the deprecation warning would explicitly state where to get it.

I do not think we have a way to make sure that withering, does not happen. Although I am not sure that is not already happening right now. I personally hope the opposite happens, not in the sense of numpy-financial growing necessarily, but in the sense of another project filling the need better. I may be wrong, but the numpy financial functions seem pretty limited, and I wonder if having them in numpy rather muffles development of tools outside of numpy.