Move machine learning code into utils class

[nya-elimu]

For better separation of concerns, move the code for calling the machine learning model from StoryBookCreateFromEPubController into its own utils class in a new ai.elimu.util.ml package. Also move the PMML file to src/main/resources/ai/elimu/util/ml, and add unit tests for covering the code in the new utils class.

---

• Depends on #1821