Support hierarchical triple systems with dual binary components (BINARY + BINARY2)

[vhaasteren]

Summary

• Adds first-class support for hierarchical triple timing models by allowing two binary delay components in a single PINT model:
  • inner orbit via BINARY with standard parameter names (PB, A1, T0, ...)
  • outer orbit via new BINARY2 with suffixed parameters (PB_2, A1_2, T0_2, ...)
• Introduces outer-orbit wrapper components (BinaryDD2, BinaryBT2) in a new pulsar_system_outer category, ordered before pulsar_system so outer delay is accumulated first.
• Preserves physical coupling automatically: the inner binary is evaluated at t_bary - (accumulated delays), so the outer light-travel delay propagates into the inner orbit evaluation epoch.
• Updates model building/validation/parfile I/O to recognize BINARY2, enforce one inner + one outer binary maximum, and round-trip _2 parameters correctly.
• Refactors binary base-component parameter access to be suffix-aware (_bp() / _hasbp()), enabling fitting and derivative support for outer-orbit parameters without changing standalone binary calculators.
• Keeps single-binary behavior unchanged and backward compatible.

Scope / Notes

• Outer-orbit support currently includes DD and BT wrappers.
• Outer suffixed orbits intentionally do not support FBn/ORBWAVE parameterizations.
• This implements coupled delay nesting (not full N-body three-body integration).

Tests

• Added dedicated triple-system tests covering:
  • component selection and ordering (outer before inner)
  • parameter namespace isolation (PB vs PB_2, etc.)
  • BINARY2 + _2 parfile read/write round-trip
  • finite residuals and non-zero outer contribution
  • propagation of outer delay into inner evaluation epoch
  • coupled solution differs from naive independent sum
  • finite/non-zero derivatives for outer parameters (fitability)
• Confirmed relevant existing binary/regression tests remain green, including single-binary workflows.

Example

BINARY   DD
PB       ...
A1       ...
T0       ...
...

BINARY2  BT
PB_2     ...
A1_2     ...
T0_2     ...
...

Note: co-developed by @coclar and Claude Code.