Celmech

* Minor bug fixes.

* Made lambidfy_kwargs an attribute of the celmech.Hamiltonian. This can be
useful when creating Hamiltonians that involve special functions.

* Fixed the setter method for celmech.Hamiltonian.values

* 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