[ENH] Packmol interface: allow user set seed?

[IAlibay]

I'm not sure if this is a feature request we really need. I'm mostly opening this for further discussion since we recently had a discussion where we thought packmol system building was randomized each time we call it.

Turns out that the default seed for Packmol is 1234567 not -1 : https://github.com/m3g/packmol/blob/master/src/getinp.f90#L33

This means that in practice calling packmol with the same inputs on the same machine multiple times should give you the same system. However, there are some cases where it might be a good idea to allow users to randomize their input systems (for example, if one configuration leads to a poor initial system - which we appear to be facing in pontibus sometimes).

[mattwthompson]

I forget if there was a reason to set the seed to a constant - maybe it should change every time?

I'm mixed on passing this through as an argument, we already have beefy function signatures. If there was a clean way to group up several options in the API I may want to explore that