NF - Add radial basis function interpolation on the sphere.

[stefanv]

No description provided.

[Garyfallidis]

I am getting this error

.............................................../home/eg309/Devel/dipy/dipy/core/sphere.py:161: UserWarning: Vertices are not on the unit sphere.
warnings.warn("Vertices are not on the unit sphere.")

..E....

ERROR: dipy.core.tests.test_sphere.test_interp_rbf

Traceback (most recent call last):
File "/usr/lib/python2.7/dist-packages/nose/case.py", line 197, in runTest
self.test(*self.arg)
File "/home/eg309/Devel/dipy/dipy/core/tests/test_sphere.py", line 181, in test_interp_rbf
v0, e0, f0 = create_half_unit_sphere(2)
TypeError: 'Sphere' object is not iterable

---

Ran 54 tests in 0.383s

[stefanv]

Added some fixes to the way triangle_subdivide constructs Spheres. Also added tests for rbf interpolation (oops, how did those get left out initially).

/cc @MrBago

[stefanv]

If you're not sure whether to merge this patch yet, should I submit a separate PR for the fixes to triangle_subdivide? They currently make it impossible to use the Sphere returned by create_unit_sphere(...).

[MrBago]

Stefan, I think I've made pretty much the same changes in my pull request. Do you mind slitting this pull request up into two and maybe we can modify mine if we need to.

[stefanv]

Done, see #45

[stefanv]

Patch also removed from this PR.