Update docs about load envelopes

I've updated the docs to explain briefly the use of the load envelopes

See merge request !2

h2_vs_hs2: added more sensors, more general form of the blade distributed parameters obtained for HAWC2 statistics, other small fixes

hawcstab2: induction output file number of columns depends on the version of HAWCStab2. Assume 38, but fallback to 34

some of the continued work on h2_vs_hs2 was done in the old prepost instead of the new wetb: updated plots, more comparison cases