Refactor single ion anisotropies

[kbarros]

• Replace set_anisotropy! with set_onsite_coupling!. 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.

[kbarros]

There are various breaking changes in this commit, but the main one is the replacement set_anisotropy! with set_onsite! (name TBD). The new function takes an explicit $N \times N$ matrix which can be built using, e.g. spin_operators or stevens_operators. DynamicPolynomials.jl becomes an optional dependency, and can provide certain symbolic analysis relevant to the large $S$ classical limit.

[Lazersmoke]

Looks great! Good to merge as-is