Let config collection populate directly?
Currently the config collection can populate itself from the module level configs and it can write its current state to file. It would be quite nice if it could populate itself directly from file (via a passed unit) so that we could try reading in the entire specified config from file without having to go through any other initialisation. This would be particularly useful for producing an executable (or even part of the usual GS2 init process) which can validate the passed input file – currently we don’t read some namelists until we’re a long way through the initialisation process. If there’s a mistake late on (e.g. in the diagnostics inputs) we can end up wasting a lot of resource. Failing early or having a validate_input exe might be quite handy.
