The class tt(VSD) (Value, Standard deviation, Distribution) stores the value,
standard deviation and distribution type of parameter values. Such parameters
are usually specified in the configuration file like this:
        verb(
          #                   value   spread  distr.                    
    parameterName:            72.9    .552    Normal 
        )

    The parameter values are retrieved by accessor members. If the
tt(Scenario's spread) parameter was configured as tt(true) then the member
tt(refresh) applies a random variation to the value parameter, which is then
returned by the tt(value) accessor. If tt(D) represents a random value from
the used distribution and tt(orgValue) is the value that was originally read
from the configuration file then tt(refresh) modifies tt(value) as
        verb(
    value = orgValue + spread * D()
        )
    which value is thereupon returned by its tt(value) accessor.

Note: if the tt(VSD spread) parameter is not specified then the default value
0 is used. In those cases the tt(VSD value) data member is not modified
when tt(Scenario spread) is specified as tt(true).
