`flux.Container.__getitem__` doesn't support list of flavors

[Sheshuk]

The flux container currently supports indexing:

flux['NU_E']

returns a new container, with only specified flavor.

But if you try to access several flavors in a list:

flux[['NU_E','NU_E_BAR']]

it fails.

This is annoying behaviour, as it prevents the user from summing over all neutrinos or antineutrinos etc.

[JostMigenda]

Right now, this accepts a list of integers (or, notably, IntEnums; i.e., something like flux[ThreeFlavor]), but it doesn’t work with a list of strings.