Costs are used by
    loop/maybefalsepositive.cc:20:
            addCost(d_costs.usingAt(screeningAge));
and
    loop/screen.cc:20:
            addCost(d_costs.screening(screeningAge, modBase->cost()));

(see also loop/writeparameters.cc:16:
            d_costs.writeParameters(out);


ModBase::costBase(keywords) extracts the the value at the costs: field of the
                            modalities, which are the screening costs.
The configuration file line read by costBase is also read by Costs,

modbase->cost() returns the value received by costBase.

Costs extracts from CT: costs: the diagnosis, M0 and M1 fields.
            Either M0 or M1 are used as treatment costs without/with
                            metastasis by Costs::treatment
            diagnosis is assigned to Costs::d_usingCosts
