Optionally allow None for missing pseudos in a family

[edan-bainglass]

At the moment, using PseudoPotentialFamily.get_pseudos() will fail for all if at least one element does not have a corresponding pseudopotential in the family. This PR aims to allow a path for partial results, returning None for those missing elements, if user opt in by way of an optional none_if_missing parameter (False by default for backwards compatibility).

This allows the QE app, for example, to do this:

[edan-bainglass]

@t-reents @mbercx no rush on this and also okay to drop. I realized in the app I can just replace get_pseudos() with a loop over get_pseudo() and try/except missing pseudos, replacing them with None. So this API is not strictly required.

I'll let you decide if this should close or merge. I'm good either way.

[t-reents]

True, that's a solution as well!

As I told you in person, I was thinking that this change is fine, as it's backwards compatible and having in mind the use-case in the app.
In my opinion, the use-case in the app is/was the strongest motivation for this change. For a normal user, I still feel that it makes sense to raise in case a pseudo is missing, since one is explicitly trying to get the pseudos for a given structure/set of elements from a PseudoFamily (even though I also see the other side to provide at least the available pseudos).

In summary, no strong opinion at my end but a preference towards the current behavior. I'd leave it up to @mbercx to make a final decision

[edan-bainglass]

Alternative solution working as expected. Closing this.