Context should include gravity fields

[ChristopherRabotin]

These should be stored with C_nm and S_nm separate from each other, a flag specifying whether the coefficients are normalized, and (tbd) the uncertainty around these coefficients (which will allow tools to generate gravity clones, and provide lossless info from PDS)

[ChristopherRabotin]

Note that this cannot be done using a clever heapless structure because there are too many items (it's a triangular number). Instead, this new format should follow a packed array approach, similar to what's in the DAF format. It may be worth making up a new such format, e.g. DAF/GRV with gravity coefficients.

[ChristopherRabotin]

I'm not convinced that a DAF is a reasonable approach here. DAF has a summary with a validity domain, unique ID, etc. But gravity fields are essentially always valid, and one chooses a degree and order. The more I think of this, the more I think it should be separate from ANISE and work only in Nyx.