Use PBEsol instead of PBE as default

[t-reents]

I just noticed that if you run aiida-pseudo install sssp, the SSSP v1.3 PBE efficiency pseudo family will be installed.

In aiidateam/aiida-quantumespresso#884, the protocols in the AiiDA-QE plugin were adjusted to use PBEsol as the default protocol. As the default protocols rely on SSSP v1.3 PBEsol efficiency, I'd like to propose to change the default in aiida-pseudo to the PBEsol functional as well. In this way, new users can simply run aiida-pseudo install sssp to get the required pseudos to run some QE calculations "out of the box". For sure, in principle one just needs to add -x PBEsol, however, I thought that this might be a bit more convenient.

Are there any reasons against the proposed change? If not, I'll open this very small PR.

[sphuber]

I am not against it per se, I am just wondering if we will have to start doing this each time that aiida-quantumespresso (or some other plugin) decides to use a different family as default. I think that this would not be really pragmatic and rather it would be up to the package to properly instruct their users to install the correct family, or allow other families to be used besides the default.