[PA-A][stokespnm] update extractpointovertime.py and averaging script.

BayesValidRox/tests/PA-A/extractpointdataovertime.py

...lidRox/tests/PA-A/models/stokespnm/averaging_stokespnm.py

...s/PA-A/models/stokespnm/ffpm_stokespnm_velocity_final.csv

...alidRox/tests/PA-A/models/stokespnm/stokes_pnm_regular_2d

BayesValidRox/tests/PA-A/models/velocity_points.csv