Essentials

This brings back ability to build variants of ParaView.

• PARAVIEW_BUILD_ESSENTIALS is a new cmake flag that can be used to control coarse-level functionality that is enabled in the build. CORE, RENDERING and CANONICAL (default) are currently supported.
• Modules added in ParaView use GROUPS to indicate which essentials build they should be included in. Further, they use CONDITION to exclude the module when it cannot be enabled due to feature or capability flags.
• ParaView settings have been renamed to follow a convention described here.
• XMLs for readers/writers/sources/filters have been split so that they are included appropriately when necessary modules are enabled. utilities.xml still remains to be split -- will do that in a follow-on commit.
• ParaView modules were restructured to be a little more coherent. Needs one more iteration, I think to clean things up but this gets us close.