Create unit tests for preprocess steps using pipeline

[junchaoxia]

There are some unit tests for individual functions performing preprocess steps such as tests/test_preprocess.py but not for the applications in pipeline as new framework.

[garrettwrong]

In the process of rebasing some work I found that preprocess functions and their pipeline application are not covered by unit tests. The tutorial does capture the issues I was expecting to see. It might be nice to have coverage of all the underlying functions and coverage in the pipeline style as suggested by Junchao.

[junchaoxia]

This issue has been addressed on PR #234. Closing now.