etl: adjust API to use an instance method over class methods

[jtigger]

As per #177, adjust the test suite to require a solution that uses an instance method instead of a class method.

(see #344 as an example and the conversation about Function Object vs. Value Object)

[stkent]

@jtigger ETL already appears to use instance methods rather than static methods, and has done for quite some time. Was there perhaps some other exercise you were thinking of here?

[jtigger]

medical marijuana? 🌱

Sorry to have created work for ya, @stkent. 😬

[stkent]

Haha, no worries, I like instant-close issues!