Create tests to catch performance regressions in specific functions

[gsarma]

We have been exploring writing tests for catching performance regressions in PyOpenWorm and ChannelWorm.

We need to first decide which functions should be tested for performance and then define reference behavior. The simplest way is to check the performance relative to some simple computation, and later we can have a more sophisticated system for tracking performance and catching regressions.

The bare minimum to close this issue is a file called, e.g. PerformanceTests.py, with a single test and docstrings.

@travs