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.
Previously we just took the optionparser.h file and stuck it right in
our source code. That was problematic for a variety of reasons.
1. It incorrectly assigned our license to external code.
2. It made lots of unnecessary changes to the original source (like
reformatting).
3. It made it near impossible to track patches we make and updates to
the original software.
Instead, use the third-party system to track changes to optionparser.h
in a different repository and then pull that into ours.
