Implement Folds for multi1d multi2d and PGamma spectra.

[rfoxkendo]

In SpecTcl, folds are conditions treated specially. Perhaps in Rust

• Either folds are folds not gates or
• Folding is handled via a trait that exists for folds/foldable spectra?

[rfoxkendo]

Commits prior to this comment implement the MultiContour and internal methods for creating it.. Sebsequent commits supprot the ReST interface to these condition types.

[rfoxkendo]

The commits above:

• Implement folds for multi1d
• Implement folds for multi2d
• Implement folds for pgamma spectra.