POL2 vector catalogues now contain more rows

A change has been made recently to the contents of vector catalogues created using the pol2map command in the Starlink SMURF package. Previously, sky positions that have a negative total intensity (I) value were excluded completely from the catalogue (such positions occur frequently in background regions because of noise). Now, these positions are included in the catalogue, albeit with a null value for the percentage polarisation (P) column and associate error column (DP).  This change simplifies the interpretation of column statistics in the background regions, since it removes the bias introduced by the exclusion of position with negative I values.

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