added default-height-unit config option
format feet values as "x ft"
added elapsed-time, ascent and descent field identifiers
renamed some of the gps::activity fields
gps::activity now loads total-elapsed-time and ascent/descent
gps::chart::point_at_x() uses total_elapsed_time where necessary
renamed gps::chart enum class values
use default units from config to initialize act::activity values
added elapsed-time, ascent and descent to act::activity
display new elapsed-time, ascent and descent fields in UI