Relax restrictions on location of headers.
Commit e6f75b9a (!634 (merged)) changed the rules for the module-Headers.cmake file, so that only headers that were directly in the module directory's SOURCE or BINARY dirs at build time were allowed. This was causing problems with the IOVisItBridge module in ParaView.
In addition to allowing more headers into the module-Headers.cmake file, the python wrapping has changed so that, if a header to be wrapped cannot be found, only a warning is generated instead of an error.