Celmech

* Addition of `poisson_series` module

* Phases out defunct `theano` dependencies in numerical resonance models. Automatic differentiation now uses `pytensor` and `exoplanet-core` packages.

* Fixes N-body simulation utilities module for compatability with rebound 4 simulation archives.

* Fixes critical bug that causes Laplace coefficients to erroneously evaluate to zero in recent versions of scipy

* Modifications for compatability with rebound version 4

* Updated the calculation of cosine amplitudes in the celmech.maps.CometMap class