Orbital set_radial and friends -- allow custom maximum R finding

[zerothi]

Describe the feature

Currently the search method for the maximum R of an radial function is based on checking when the radial function is strictly 0.

However, this is pretty bad for exponentially decaying functions. So better to retain a "contained" fraction.

I would propose we do something like this:

*Orbital(R = -0.9999) # contains locates minimum r where radial(r) ** 2  >= max(radial(r)**2) * 0.9999
*Orbital(R=10) # retains a maximum radius of 10 Ang

I think this would be a better approach and should be simpler for end-users to detect problems.