ENH: Make group_var_ use Welford's algorithm.
closes #10448 

This PR reimplements the Cython functions `group_var_float64` and `group_var_float32` to use Welford's algorithm, rather than the sum-of-squares method, which is numerically unstable. This came up in #10448; see also #10242 for more discussion.

@jreback Would you be willing to give this a review? I can probably tackle `_nanvar` at some point too, but not before the weekend.
 looks good. pls add:
- release note in 0.17.0 whatsnew (you can put in bug fix section)
- add a test that should match before this PR (to assure there isn't a calculation regression), e.g. something that isn't an edge case 
- squash
 @jreback Thanks for your feedback. I added some tests and squashed everything onto one commit. I also took the liberty of fixing up the "closes" trigger in the PR description: this addresses #10448 --- I have some ideas for #10242 but would like to address that in a separate PR.
 couple of style issues, otherwise looks good. ping when pushed / green
 @jreback Done and Travis shows green. Thanks for the review!
 thanks! this looks gr8!
