A generic source processing stage

Participants were nonplussed by having to be explicit about C and Fortran preprocessing. There was a feeling that there might be a generic "preprocessor" source processing stage which used some means (possibly file extension) to dispatch files to the correct processor. This might be extensible so additional source processing stages, e.g. PSyclone or CLAW, could also be handled.

The issue names no files, tests, or entry points. Start by reviewing how the existing C and Fortran preprocessing stages are selected, then determine the scope of generic dispatch and extensibility; done should include a defined approach that supports current processors and possible additional stages.

Written by the indexing model from the issue text.