Package simplification

[PeterDSteinberg]

The following changes I think will make the code layout more intuitive:

• [ ] Move the contents of elm/preproc to within elm/sample_util
• [ ] Get rid of elm/preproc as a subpackage in setup.py, elsewhere if needed
• [ ] Move most or all of the contents of elm/pipeline/sample_pipeline.py to elm/sample_util/sample_pipeline.py (a new file)
• [ ] Search and replace for changes required by above items, ensuring to check for functions that are not directly imported by provided as callable strings like "elm.pipeline.sample_pipe:some_func"

[PeterDSteinberg]

cc @brendancol and @stuartarchibald I'll fix this issue soon. It is somewhat related to your issues, but shouldn't hold you up.