Scenario: spread; true causes the parameters to vary by chance, using a
specific distribution. 'spread:' is either active or not active during the
complete simulation run.

Specifications that may use spreading specify a distribution producing values
that modify the original value or randomly generate new values (used by the
Beta distributions)

Parameter variation is activated by Scenario's constructor.

Scenario's constructor may call Random::useVary(),
                                Distribution::useVary   [ Q ]

Scenario::vary() returns true if 'spread: true' was specified

Loop::iterate calls
        Modalities::vary()
        TumorInfo::vary()

Random::reset()  clears the caches of the distribution generators
                 (like d_uniform)

Modalities::vary()  calls (if active):
        CT::vVary()
        Mammo::vVary()
        Tomo::vVary()
        MRI::vVary()

CT::vVary()
        VSD::vary()

Mammo::Vvary() calls
        VSD::vary()
        AgeGroupVSD::vary()

Tomo::vVary() calls
        VSD::vary()
        AgeGroupVSD::vary()

MRI::vVary() calls
        VSD::vary()

TumorInfo::vary() calls
        Beir7::vary()
        Growth::vary()
        Incidence::vary()
        Survival::vary()

AgeGroupVSD::vary() calls
        VSD::vary()

Beir7::vary() calls
        VSD::vary

Growth::vary() calls
        VSD::vary

Incidence::vary() calls
        VSD::vary

Survival::vary() calls
        VSD::vary

VSD::vary() calls
    d_value = d_dist.value(d_orgValue), determining the varied value returned
                                        by VSD::value()

VSD defines the distribution by its enum VaryType:
        enum VaryType
        {
            VARY_MEAN,
            VARY_NONNEG,
            VARY_PROB,
            VARY_BETA_MALE,
            VARY_BETA_FEMALE,
        };

VSD's constructor is called in
    beir71.cc
    ct1.cc
    mri1.cc
    agegroupvsd1.cc
    vsdparameters1.cc
    vsdparameters2.cc
    growth1.cc
    setincidence.cc


Distribution's constructor sets d_vary to the function handling the variation.
E.g.,
    varyMean, computing a variation around the configured value(orgValue)
    varyBeta, returning a generated value from its beta-distribution
              (orgValue not required and not used), calling Random::betaVary

TODO: but only if variation is active.
