Improved handling of zeros in radial functions

[trettberg]

Radial functions bn often exihibt zeros at kr==0.
Previously a terrible hack was used to handle these.

This PR introduces a function replace_zeros to do this in an explicit and more graceful way.
It is automatically invoked by spherical_pw, spherical_ps, via a decorator.

The monofrequent case works transparently,
closes #4

Also, the kr==0 case in radial.weights and radial.circ_weights was fixed.
However, this is not yet done for point and line sources, see #9.