Imap-Lo systematic error calculations

[vineetbansal]

The IMAP-Lo team has identified some problems with the existing calculations of systematic uncertainties in the l2b products:

• The Cntr_E_unc values in the v4 geometric factor csvs are in [eV]. They should be divided by 1000 to convert them to [keV] (and make the comparable to the v1 values).
• The headers should be as follows (these match already and thus nothing needs to be changed here):
  • esa_mode, incident_E-Step, Observed_E-Step, Cntr_E, Cntr_E_unc, GF_Trpl_H, GF_Trpl_H_unc_minus, GF_Trpl_H_unc_plus
• Use geometric mean of minus/plus systematic error values for overall systematic error (for ena_intensity_sys_err, bg_rate_sys_err, bg_intensity_sys_err), not arithmetic mean.
• The *_plus systematic error for a variable is calculated as:

value * dg_minus / (gf - dg_minus)

• The *_minus systematic error for a variable is calculated as:

value * dg_plus / (gf + dg_plus)