This mechanism sets up CMake variables that allow a user to select which
modules/libraries to create. Dependencies will be tracked down to ensure
that all of a module's dependencies are also enabled.
The modules are also arranged into groups.
Groups allow you to set the enable flag for a group of modules at once.
Thus, if you have several modules that are likely to be used together,
you can create a group for them.
This can be handy in converting user-friendly CMake options (such as
`VTKm_ENABLE_RENDERING`) to the modules that enable that by pointing to
the appropriate group.
When you used the `AddHelp` option to `Initialize`, it added a
`--vtkm-help` option and `-h`, but not `--help`, which was weird. It
also avoided adding `--vtkm-help` when `AddHelp` was not used, but did
print out a usage statement under other circumstances.
This changes the behavior to add `--vtkm-help`, `--help`, and `-h` when
`AddHelp` is on and only `--vtkm-help` when it is off.
