Add spin_pols_iter to iterate over a process' spin/pol combinations

[AntonReinhard]

As the title says, this adds an iterator to yield all possible combinations of spins and polarizations allowed by a process' set spin_pols(). For example:

julia> using QEDbase; using QEDcore; using QEDprocesses;

julia> proc = ScatteringProcess((Photon(), Photon(), Photon(), Electron()), (Photon(), Electron()), (SyncedPolarization(1), SyncedPolarization(2), SyncedPolarization(1), SpinUp()), (SyncedPolarization(2), AllSpin()))
generic QED process
    incoming: photon (synced polarization 1), photon (synced polarization 2), photon (synced polarization 1), electron (spin up)
    outgoing: photon (synced polarization 2), electron (all spins)


julia> for sp_combo in spin_pols_iter(proc) println(sp_combo) end
((x-polarized, x-polarized, x-polarized, spin up), (x-polarized, spin up))
((y-polarized, x-polarized, y-polarized, spin up), (x-polarized, spin up))
((x-polarized, y-polarized, x-polarized, spin up), (y-polarized, spin up))
((y-polarized, y-polarized, y-polarized, spin up), (y-polarized, spin up))
((x-polarized, x-polarized, x-polarized, spin up), (x-polarized, spin down))
((y-polarized, x-polarized, y-polarized, spin up), (x-polarized, spin down))
((x-polarized, y-polarized, x-polarized, spin up), (y-polarized, spin down))
((y-polarized, y-polarized, y-polarized, spin up), (y-polarized, spin down))

julia> length(spin_pols_iter(proc))
8

The above is also a jldoctest.

As a side-note I also added an alias of SyncedPol to SyncedPolarization.

The code is not incredibly concise and also not incredibly fast, but for the reasonable cases that I tested @benchmark reports well under 1ms. Since I don't think this iterator would be the critical path of anything this should be fine.
The only problem I could see is that due to everything using Tuples in its arguments, the compile time is relatively large. If this becomes a problem we could change it to using Vectors instead, likely trading some runtime for much better compile time.

Fixes #107