configuration distinction between "fitting" and "project" modes

[pearsonca]

We have typically used AbcSmc in two modes: "fitting" and "project". However, there are not generally unambiguously distinguishing configuration file features. Nor is there a core abc executable that will take a -f|--fitting vs -p|--project flags.

If we could know that we were in "project" mode, then lots of boilerplate options could be removed and/or set to defaults for the configuration file.

For example, all of the predictive prior / PLS fraction keys are irrelevant (maybe even error inducing). num_samples could be defaulted all combinations.

[pearsonca]

Heuristically:

• if there are no "prior" parameters => assume project mode
• ... no smc_iterations => assume project mode
• ... none of predictive_prior_(size|fraction), pls_training_fraction => project mode
• ... posterior_database_filename => project mode?

[pearsonca]

If we introduced mode key, should certain keys yield errors / warnings?

Probably warnings - would support future use where mode is settable from command line invocation, and would override the configuration file?