[Depends on #3703] Direct and Persistent Interfaces to SCIP

[michaelbynum]

Fixes #849 .

Summary/Motivation:

The primary purpose of the PR is to port the direct and persistent interfaces for SCIP from #3200 to contrib/solver. I worked off of the branch created by @Opt-Mucca, so all of their commits are preserved. I will close #3200 shortly.

Changes proposed in this PR:

• Create direct and persistent interfaces to SCIP
• Expand the API of the ModelChangeDetector in contrib/observer slightly

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