BUG - Epoch time should be TDT instead of TDB

[subagonsouth]

Description of the issue

Currently, the met_to_j2000ns function is being used to compute values stored in CDF files with the CDF datatype CDF_TIME_TT2000. The implementation of met_to_j2000ns provides TDB nanoseconds but CDF_TIME_TT2000 is Terrestrial Time nanoseconds.

This ticket needs to fix that bug and add a function for converting TT nanoseconds to floating point TDB seconds (ET) for use with SPICE.

Steps to reproduce the issue

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Expected behavior (What should happen)

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Actual behavior (What does happen)

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Code Snippet:

Additional notes, affected areas, and suggested fixes

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