Support where filters in classifications

[etiennedi]

Background

Weaviate allows a set three different where filters in classifications, but the client currently does not take any arguments.

Goals

• .withSourceWhere() is added to the classification builder
• .withTrainingSetWhere() is added to the classification builder
• .withTargetWhere() is added to the classification builder

[dirkkul]

will not do this - classification is rarely used