Add modular VTK build system

Add module.cmake, CMakeLists.txt, and other build system files.

The modular VTK build system is not yet mature.  The monolithic build
files had a lot of infrastructure.  The modular build files reproduce
much of the functionality but some features will need to be restored
later.  Document status and tasks in "TODO-Modularization.txt".

Co-Author: Marcus D. Hanwell <marcus.hanwell@kitware.com>
Co-Author: Chris Harris <chris.harris@kitware.com>
Co-Author: Brad King <brad.king@kitware.com>
Co-Author: Nikhil Shetty <nikhil.shetty@kitware.com>