Changing lambdify to use numpy by default

[Aaron Meurer]

Right now if you want to use lambdify with numpy you have to
explicitly pass 'numpy' as the third argument, like lambdify(x, expr,
'numpy'), as it defaults to math otherwise (technically it defaults to
math but falls back to numpy if the function isn't in math).

I've got a PR to change this so that it defaults to numpy always
(unless numpy isn't installed of course) at
https://github.com/sympy/sympy/pull/11468.

If you have an opinion on this one way or another I'd love to hear
your comments on the pull request. I know lambdify is one of the most
used SymPy functions, so I don't want to take a change like this
lightly.

Aaron Meurer