ENH: add tolerance argument to .sel and .reindex

[shoyer]

.sel and .reindex now support the tolerance argument for controlling nearest-neighbor selection:

In [5]: array = xray.DataArray([1, 2, 3], dims='x')

In [6]: array.reindex(x=[0.9, 1.5], method='nearest', tolerance=0.2)
Out[6]:
<xray.DataArray (x: 2)>
array([  2.,  nan])
Coordinates:
  * x        (x) float64 0.9 1.5

This feature requires pandas v0.17 or newer.

[shoyer]

@jhamman I think I want to merge this and then issue a new release. We are doing more harm in waiting for ReadTheDocs at this point. Better to get the fixes out even if we can't make the stable docs yet.

[jhamman]

What error do we get if we use the tolerance keyword with pandas 0.16? Do we need to catch that ourselves?

[shoyer]

You should get a TypeError, since the argument does not exist. I'll double check, though.

[jhamman]

I'm fine with merging this now. I agree we need to get this release out, with or without read the docs support.