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.
The purpose of the TestBuild infrastructure was to confirm that
VTK-m didn't have any lexical issues when it was a pure header
only project. As we now move to have more compiled components
the need for this form of testing is mitigated. Combined
with the issue of TestBuilds causing MSVC issues, we should
just remove this infrastructure.
