Use a different proxy for determining fast survey intervals on THEMIS-A when FGM is turned off

[jameswilburlewis]

With the FGM turned off, everything gets marked as slow survey because there is no FGL data when the instrument is shut off.

For example, eESA is in mode 0x0101 for slow survey, and mode 0x0301 for fast survey, which might be one way to check.