Improvements to VTK Tcl - SWIG Tcl integration
Fixes #27 (closed) by creating a CMake var _tcl_lib_prefix
(Currently sets to lib
when CMake version <= 3.9, else blank)
Fixes #26 (closed) by getting VTK & SWIG Tcl wrapping to work together so can read via vtkUnstructuredGridReader
plus vtkFullMonteMeshFromUnstructuredGrid
Tested on BigMac with CMake 3.8 and 3.10.2.