Make output directory constant, detect changes that invalidate cached output files

[JulianKemmerer]

Currently the tool makes a new output directory for each run.

That is unless the user specifies a fixed --out_dir. However, it then becomes the users responsibility to 'clean'/remove cache files in the output directory when making changes to the design... because the PipelineC does not automatically...

The issue is that the pipelinec tool caches things in the output directory, but has no mechanism to know if the cache is stale...

make based approaches have been suggested...