Modularize validate method for multi-table synthesizers

[frances-h]

Problem Description

Currently, we cannot easily extend validation for multi-table synthesizers because most of the functionality is contained in the public BaseMultiTableSynthesizer.validate method. We should modularize the validate method so it can be extended more easily.

Expected behavior

In order to support additional validation functionality, we should move the existing validation functionality out of the validate method and into the helper method _validate. The validate method should just call _validate.

Additionally, in the preprocess method, validation of the data should occur before the _transform_helper method is called on the data.