nn_radius improvements; setup with conda transparently

[robbmcleod]

Hi,

Two suggested changes:

1. Some additional checks for nn_radius to allow [1,2]-shaped queries, which to me are the natural shape for such queries, and also ensures that params['checks'] is high enough to accomodate max_nn.
2. In setup.py, try to infer if Conda is in use and set the appropriate environment variables in that case. Otherwise it doesn't work out-of-the-box.

I can squash or separate the PRs if you want, I was a bit lazy in committing.