Adds the normalised, dashboard-facing effort metrics (avg_fidelity,
constancy, avg_hours_per_day, avg_visits_per_day, hours_per_trip) to
an H3 effort table. Shared by the all-fleet and per-gear/country exports in
export_pds_spatial() so both use identical formulas.
constancy is the share of the available days on which the cell was fished.
Rows are per (cell, year), so the denominator is the days of that row's
year that fall inside the study period – not the length of the whole series,
which would compare one year's activity against three years of calendar and
put every value in the third decimal. A cell fished every available day of
its year scores 1, and years are comparable with each other: the first and
last years of the series are measured against the part of them the study
actually covers.