Modify EventPositionUncertainty plugin to work with full FDC-propagated uncertainty contours

[juehang]

The relevant lines of code are here:

straxen/straxen/plugins/events/event_position_uncertainty.py

Line 316 in e222b25

result["r_position_uncertainty"] = result["s2_r_position_uncertainty"] * scale

Currently, this plugin uses the field-distortion correction of the best-fit position to scale the uncertainties. If we propagate the entire contour using the FDC, then this would lead to an inconsistency between the contour and the r/theta uncertainties. Propagation of the contour is not an option right now, so this is not yet a problem, but this fix is a prerequisite for uncertainties to be fully propagated through the FDC correction.