Inplace ops

[hameerabbasi]

Closes #80

Leverages the existing __array_ufunc__ implementation without needing too much extra code.

[codecov-io]

Codecov Report

Merging #146 into master will increase coverage by 0.64%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #146      +/-   ##
==========================================
+ Coverage   95.77%   96.42%   +0.64%     
==========================================
  Files          10       10              
  Lines        1183     1174       -9     
==========================================
- Hits         1133     1132       -1     
+ Misses         50       42       -8

Impacted Files	Coverage Δ
sparse/coo/umath.py	96.9% <ø> (+0.47%)	⬆️
sparse/coo/core.py	94.63% <100%> (+0.92%)	⬆️
sparse/coo/indexing.py	100% <0%> (ø)	⬆️
sparse/coo/common.py	97.04% <0%> (+0.92%)	⬆️
sparse/dok.py	95.32% <0%> (+1.74%)	⬆️

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[mrocklin]

This looks pretty slick :)

Might want to test something like the following as well:

np.sin(x, out=x)

[mrocklin]

I've seen people do things like the following to ensure length

(out,) = out

[dhirschfeld]

Might necessitate a docs update?

sparse/docs/operations.rst

Line 23 in 8f2a9ae

However some operations are not supported, like inplace operations,

[hameerabbasi]

Docs updated, warning added to docs that "in-place ops aren't really in-place".

Supported out kwarg for all ops now, including round and COO.sum, etc.

[hameerabbasi]

cc @mrocklin I think this is ready for a final review. :-)

If ready, please do a rebase+merge, I've kept the changes separate.

[hameerabbasi]

Removed some unnecessary code for astype -- It doesn't have an out argument. It was what was causing coverage to fail.

[mrocklin]

Merging. Sorry for the delay in review. Thanks @hameerabbasi