Report default values of parameters in OMSimulator --help output

[casella]

OMSimulator has many flags, which are output by calling OMSimulator --help. These flags all have default values, set by oms::Flags::setDefaults(), which are however not visible from the documentation.

We should make sure that OMSimulator --help calls [oms::Flags::setDefaults() and then displays the actual default values in the on-screen documentation. Then, we should updated the online documentation accordingly.