Refactoring the DRS schema

[daisieh]

I realized that I created many extra schema components related to DataDrsObjects and ContentsObjects that are not actually different from each other and just confusing to look at or comprehend.
Fundamentally, it is simpler to consider it this way:

• DrsObjects have ContentsObjects in them; all these do is to hold the link to the linked DrsObjects that are the actual "contents" of the object. There don't need to be subtypes of ContentsObjects.
• DrsObjects that describe data files are all the same sort of DataDrsObject. They don't have ContentsObjects but do have AccessMethods.

This version consolidates the following:

• AnalysisIndexContentsObject, AnalysisDataContentsObject, AnalysisContentsObject, and ExperimentContentsObject into one ContentsObject
• AnalysisDataDrsObject and AnalysisIndexDrsObject into DataDrsObject

You can check this by recomposing drs on a clean build and running the integration tests to see that ingest is not affected.

I can also confirm that I dumped the drs database on a build without these changes and then a build with these changes, and both were identical.

[mshadbolt]

Works with clean rebuild and doesn't seem to affect any functionality

Seems like a good simplification