Allow likelihood definition .yaml to pull from other sources

[kdund]

While the likelihood .yaml is most often a great place to store detector numbers, in some case it could also be good to store these (say expectation values for each source) in some centrally managed fconfig file.
For those cases, we could consider allowing the config to say "take this number from this other file" if possible and/or allowing just-in-time filling in of these numbers at initialisation?