Add multipolar variants of iadd and integrate, also for full grid

[tovrstra]

This can be done in the same way as in the hold horton.grid, by providing a list of GridFunc objects, in the canonical order of the corresponding spherical harmonics. Recursion relations for computing the spherical harmonics efficiently can be found in horton 2.x

[tovrstra]

A whole grid integration should be carried out when cutoff==NaN.

[tovrstra]

Also consider a variant of integrate that only integrates over angular degrees of freedom w.r.t. a custom center.