Use SPICE to define J2000 and IMAP epoch

[subagonsouth]

When reviewing code changes after adding the spice driven met_to_j2000ns function, I noticed the import of J2000_EPOCH defined in imap_processing/cdf/utils.py. That definition is used in imap_processing/glows/l1a/glows_l1a.py. That value should be defined using spice if it is needed in the GLOWS and MAG code.