Add option to install family from files on disk

[sphuber]

Currently aiida-sssp only allows installing an SSSP configuration by downloading it from the Materials Cloud archive. This can be quite limiting for example when the resources are not available or the machine does not have internet connection. We should look into providing the option to install from resources already on disk. The question is what level of validation we implement. Do we allow non-official SSSP configurations and just any UPF family? Should we make a separate Group subclass for these, i.e. a UpfFamily with the SsspFamily being a subclass of this adding additional restrictions and validations?