Expose vis it bridge

This aims at exposing VisItBridge for external vtk modules, specially in the context of a ParaView plugin.

As a main change, the bridge logic is in its own module. We now have the following:

• the module VisItLib is the core of VisIt, in the Library directory,
• the module IOVisItBridge contains only the actual bridge part, in the AvtAlgorithms directory with the abstract vtk readers, *.in files and the VisItReader.cmake script
• the module IOVisItReaders contains the readers, in the databases directory

Related changes in ParaView are here: paraview!4462 (closed)