SPICE - Error in computing filename date string in write_cdf

[subagonsouth]

Algorithm Description:

While working on SPICE time functions, I discovered that the implementation in imap_processing/cdf/utils.py::write_cdf for generating the date string from the zeroth element of the Dataset epoch coordinate is incorrect by 5 seconds due to the occurrences of 5 leap-seconds since the J2000 epoch. This ticket it to evaluate and figure out a solution. Ideas:

• Hard code 5 second offset into calculation. The advantage of doing it this was is that writing the CDF will not require any SPICE kernels be furnished. The disadvantage is that it will be wrong if/when another leap-second occurs.
• Use SPICE. The advantage is it will always be correct assuming the correct leap-second kernel is furnished.