Feature request: dynamical model with corrections for general relativity

[mkelley]

This is a request for

• a new feature
• an enhancement to existing sbpy functionality
• somethings else: [explain here]

The requested changes will be implemented by

• me
• the sbpy developers
• other
• any of the above

High-level concept
Near the sun, GR effects can be important in dynamical models. Could it affect syndynes/synchrones? Let's add a DynamicalModel with GR corrections.

Explain the relevance to sbpy
Because comets have been observed in the near-Sun environment.

Proposal details
TBD

Example (pseudo-)code
TDB