Minor cleanup in nengo.utils
Add test for nengo.utils.tuning_curves().

Also fix nengo.utils.tuning_curves() for non-scalar ensembles.
Add tuning_curves_along_pref_direction().
Implement meshgrid_nd function.

Numpy's meshgrid did not support more than two dimensions
or ij-indexing before version 1.7.
Merge tuning_curves functions. Add more tests.

Changes in regard to the tests:

- Additional tests for LIF neurons (so far only direct mode neurons
  were tested).
- Easier plotting of tuning curves with dim > 2.
Split tuning_curves again in two functions.
Add 1d tuning curve function for convenience.
Respect radius in tuning curve calculation.
Added tuning_curves notebook

This helped me understand how to use the tuning_curves
and response_curves functions. Guessing it will be useful
to others as well!
Removed tuning_curve_1d function

Instead, I changed the return value of `tuning_curves`.
Now, it is possible to unpack the result of `tuning_curves`
to plt.plot in the 1D case, and the 2D case is only a little less
clean. See the notebook diff for details.