Recompute old activities to get new metrics

[Daniel Besse]

W'exp, TISS, low/high/peak, and focus are missing for a lot of people.

Adapt update_all_empty_TSS (in Activity.php) to update all activities with streams != NULL but focus (or any of those) = NULL.

Once done activate solution in issue #136 and resolve it.

[Jean-Claude Besse]

allow updating of missing metrics (resolve #141) and plots moving avg instead of total avg in velocity graph

→ <<cset 6dfab25af605>>