The class tt(Loop) is the `workhorse' class. It performs the simulation as
specified by the tt(Scenario) object which is passed to its objects when they
are constructed. The class tt(Loop) uses many other classes, most of which
encapsulate parameters specified in the configuration file. Those classes read
configuration file parameters and prepare these parameters for their use by
tt(Loop).

The constructor of a tt(Loop) object defines the following objects:
    itemization(
    itt(d_costs): a tt(Costs) object providing various costs
        parameters (cf. section ref(COSTS)). This object does not provide the
        costs associated with various modalities. Those costs are directly
        stored at the modalities themselves; 
    itt(d_densities): a tt(Densities) object providing the breast-densities
        parameters (cf. section ref(DENSITIES)); 
    itt(d_modalities): a tt(Modalities) object providing the details about
        which screening modalities are used in a specific simulation
        (cf. section ref(MODALITIES));
    itt(d_screening): a tt(Screening) object providing screening parameters
        (cf. section ref(SCREENING));
    itt(d_tumor): a tt(Tumor) object handling the simulation related to the
        occurrence of tumors (cf. section ref(TUMOR)); 
    itt(d_tumorInfo): a tt(TumorInfo) object providing tumor-related
        configuration parameters (like incidence, growth and survival
        parameters. Cf. section ref(TUMORINFO)); 
    )

It also defines a tt(Random) object (tt(d_random)), generating random numbers
(cf. section ref(RANDOM)). 







