Add optional sun grazing flyby to inner planet program (or a different program if too difficult)

[Clayell]

Contract to get a probe decently beneath mercury's perihelion, so let's say 40 Gm (Mercury's perihelion is about 46 Gm).
Would have done this as a PR myself but could not find out how to set the sun as the parent body for the contract.
This contract might be too difficult for the inner planet program, so it could be moved to a later program if needed. A new program just for this isn't needed though.