test_eos does not work for all networks

[maximumcats]

test_eos explicitly uses "solar" metallicity, and so it will not work for networks that don't have ih1 or ihe4 defined. We should generalize this so any net can use it.

[zingale]

do you have an idea in mind on how to generalize this?

I can just make the composition evenly distributed through the different species.

[maximumcats]

Yes, evenly distributed is the easiest, I am fine with that.