Implement self-convolution for multi-phonon contributions

This provides an inexpensive isotropic approximation to order 2, and is generally the only feasible route to quantum orders 3+.

*Duplicate of original issue at https://github.com/pace-neutrons/abinslib/issues/33*

The issue requests self-convolution for multi-phonon contributions but names no files, tests, or entry points. Start by reviewing the superseding original issue at https://github.com/pace-neutrons/abinslib/issues/33 and locating the existing order-2 or multi-phonon calculation path. Done should provide the stated isotropic approximation and cover the relevant quantum orders with tests.

Written by the indexing model from the issue text.