ENH: Allow abs to work with PandasObjects

[jtratner]

Add __abs__ method so that you can use the toplevel abs() with
PandasObjects. I note there aren't that many existing test cases for
abs. At some point we might want to think about that.

Also related #4819

cc @Thisch

[jtratner]

if no objections, will merge this soon. Entirety of the change is adding this:

def __abs__(self):
    return self.abs()

[jreback]

this is fine....just an oversight, I think < 0.13, abs(a_series) worked because numpy defined abs

[cpcloud]

Yep numpy does define abs. Looks good.

[jtratner]

thanks for pointing this out @Thisch - now all pandas objects support abs and your original code should now work.

[jtratner]

what the heck? I added release notes to this and pushed them to Github before merging, but they didn't end up in this PR. weird. Worth committing separately @jreback ?

[jreback]

ha....just was noticing that...no...you can throw them in next time you merge something

[cpcloud]

no test for poor-old Series 😢?

[jtratner]

It gets tested in test_panel (weirdly). If we want to add abs tests, we should do it to test the abs method...this is basically just adding some sugar :)

[cpcloud]

ah ok yep i c now