GDP: Use private_data for all mappings between original and transformed components

[emma58]

Fixes # .

Summary/Motivation:

This moves all of the mappings between original and transformed things in pyomo.gdp onto the new Block private_data infrastructure. It's very pretty.

Changes proposed in this PR:

• Uses private_data for all the mappings in pyomo.gdp
• That changes a couple tests from KeyErrors to GDP_Errors, but that's the most significant change you can see from the outside.

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