Gravity fields for spherical harmonic computations

[ChristopherRabotin]

It should be possible to use ANISE to store gravity fields, e.g. EGM2008. At the moment, software will typically read this data from a text file found on NASA PDS in a specific format.

Text formats are great for data that is typically read or edited by humans. This is not the case for gravity field information.

Gravity field information contains the C_nm and S_nm coefficients and may optionally contain the standard deviation of those coefficients. This standard deviation data can be used to generate gravity clones, often used in orbit determination to see how small errors in the gravity field may affect the OD solution.

Storing this data as a packed structure will be significantly more efficient, and would prevent memory allocation to read said data.

Moreover, it should be possible for several gravity fields for the same object to be stored in the same file. This will allow for tools like Nyx to quickly switch between gravity fields while reading the same file.