METRIC/IMPERIAL trouble series: ALT, CALT, VOSC and PCO

[alex-hhh]

The following series are always kept in metric mode: "alt", "calt", "vosc", "pco" (maybe others?) so all the plots use metric values. See [1] for the meaning of these series.

Summary values (e.g lap average) are displayed as imperial when that system is selected.

Trouble is that summary data when detecting various intervals (intervals.rkt) is expecting them to be always in metric (as the summary values are actually converted for imperial for display). We need to do something similar to the way we handle distance and speed ("dst" -- always metric, "distance" -- metric or imperial; "spd" -- always metric, "speed" -- metric or imperial)

[1] https://github.com/alex-hhh/ActivityLog2/blob/master/doc/session-df.md

[alex-hhh]

The altitude/corrected altitude series is now fixed, PCO and VOSC still have this problem.