Add M1 transitions

[neil-glikin]

Two parts to this:

1. Calculating the transition matrix element. This will manifest as a new case (if multipole == "M1") in the function matrix_element in ions.jl.
2. Extending the loop over allowed transitions in hamiltonians.jl to include transitions between sublevels that may be coupled with an M1 transition.

[neil-glikin]

For the second part: hamiltonians.jl determines its list of possible transitions to loop over from the function subleveltransitions in ions.jl. Currently this does not return any pairs of energy sublevels within the same energy level. This portion may simply come down to including those pairs in the return result of subleveltransitions. However it's also possible that we want to avoid doing this unless we need to (e.g. only if a "Microwave" is present in the Hamiltonian).

[neil-glikin]

To do on branch M1-transitions:

• [x] Test math of new matrix_element method for M1
• [ ] Modify subleveltransitions in ions.jl to search for M1-allowed transitions within energy level manifolds
• [x] Change (most? all?) references to Lasers in other code to refer instead to LightFields
• [x] Decide how to deal with pointing parameter since we are now possibly dealing with Microwaves. Some possibilities:
  • Microwave types also have a pointing parameter, which automatically (and immutably?) becomes 1 for all ions upon being introduced to a trap
  • Microwave types do not have a parameter pointing, and instead, code that uses LightFields ensures that Microwaves point at all ions in some other way
• [x] Decide where electron g-factor belongs