- Replace `set_anisotropy!` with `set_onsite!`. The new function expects
  a matrix built from `stevens_operators` or  `spin_operators`. Provide
  `print_stevens_expansion` to decompose polynomials of spin operators
  into a linear combination of Stevens operators.

- Make DynamicPolynomials an optional dependency. This provides symbolic
  functions `print_classical_stevens_expansion` and
  `print_classical_spin_polynomial`.

- Functions to create tensor products of operators.