Made visualization, io and outofcore tools optional

Added so visualization, io and outofcore tools can be deselected/selected for building.

This is to minimize required projects for just compiling core libraries.

Not sure if there should be created a guideline for tools? as there is the common tool folder and apparently subfolders in ie. visualization, io and outofcore - mayby they should be moved instead?

note: converted a few tabs to 4 spaces.
I would propose to name it BUILD_OUTOFCORE_TOOLS to be consistent.