support new metric argument for nn2()

[krlmlr]

• calls are forwarded to RANN1
• documentation in the main README file

Fixes #3.

Closes #4 (=contains it).

[krlmlr]

Now, get_nn2_func uses an environment for lookup, the median run time jumps around 10 microseconds per call.

> microbenchmark(RANN:::get_nn2_func("manhattan"), times = 100000)
Unit: microseconds
                             expr   min    lq     mean median     uq      max neval
 RANN:::get_nn2_func("manhattan") 4.771 6.067 10.57319  7.057 14.842 1706.912 1e+05

[jefferis]

The use of the environment is a nice way to speed things up. My only comment now would be: should we just bypass worker_nn2 for Euclidean distance? That way the implementation is essentially unchanged for the standard Euclidean distance.

Either that or prefill the environment in onLoad

[krlmlr]

For a long-term solution, I think it's best if the master and master-L1 branches are as similar as possible. I'll find a solution with minimal penalty for the "default" case.

[jefferis]

So the conclusion for the time being is that we simply refer to your new RANN1 package in the docs as implemented in your #10. This PR has therefore been superseded so I am closing it (even though some of the approaches you tried here were interesting).

[krlmlr]

Fine with me, thanks.