Fix bug in fluxcalib point_source_correction

The calibration vector (ccalibration) is normalized by the fiber aperture correction factor (point_source_correction), but its inverse variance was normalized. This PR fixes that.

Author: rongpu

py/desispec/fluxcalibration.py

@@ -1349,6 +1349,7 @@ def add_margin_3d_dim2(iarray,margin) :
 
     #- Apply point source flux correction
     ccalibration /= point_source_correction[:,None]
+    ccalibivar *= (point_source_correction[:,None])**2
 
     log.info(f"{camera} interpolate calibration over Ca and Na ISM lines")
     # do this after convolution with resolution