Correction on phytoplankton components in rivers

[vdibiagio]

https://github.com/inogs/ogstm_preproc/blob/373938d61e221564597fc957bc3a57cf93cc1452/BC/EFAS/rivers.xml#L142C1-L145C30

Currently the sum of Pxl components in rivers is 2 (which refers to mean values measured by satellite in Po river, then spread to all the others rivers), but @gcossarini suggested to put the sum equal to 4 (after some river runs).
The doubt is about the other P1y, P2y, P3y, P4y components (with y = c, n, p, and s only for 1) @plazzari @gcossarini

[vdibiagio]

After the run neccton of August 2025 (where the sum of Pxl components was equal to 4), @gcossarini suggested to modified the values of phytoplankton components in rivers such that the sum of Pxl components becomes equal to 5. This change can be then merged into the main code.