FrequencySimulationResult, FrequencySimulation, and basis order

This is just to debate out current types.

Am not sure how useful is the type `FrequencySimulation` now. Essentially it is not just an added step were the user always has to call:
```julia
sim = FrequencySimulation(particles, source)
result = run(sim, x, ω)
```
Why not just have:
```julia
result = run(particles, source, x, ω)
```
how is `FrequencySimulation` useful?

Also for debate, I think `FrequencySimulationResult` should have a field `basis_order::Vector{Int}`. I think by having the field `basis_order` it makes it easier for new users to discover this option, rather than just being keyword of `run`. Second, even if the used doesn't specify `basis_order` it can still be very important to know and keep a record of what the basis order was. On the other hand, if `FrequencySimulationResult` didn't have the field `basis_order` that would allow us more freedom to, for example, have a different basis order for each particle, depending on it's radius....