Define pixel size and override radii in FIMTrack v2 csv files

Reconstructed outlines for tracks from FIMTrack v2 table.csv files can be very noisy due to highly variable radii (both in space and time). These radii can be zero or abnormally large.

For visualization purposes, we need to override the radius_1, radius_2 and radius_3 rows and use constant values instead.

The larvatagger.jl import command could feature an argument --override="radius_1:<val1>,radius_2:<val2>,radius_3:<val3>".

EDIT: as expected, larvae exhibiting torturous shapes seem to be segmentation errors in most cases. We recycle this issue to mention the introduction of the --pixelsize argument to larvatagger.jl import.

Pixel size is passed in micrometers and translates into space coordinates in millimeters.