Composite overshoot

[matthiasfabry]

add functionality to model a step + exponential overshoot.

you can enable with:

      overshoot_scheme(1) = 'step+exponential'
      overshoot_zone_type(1) = 'burn_H'
      overshoot_zone_loc(1) = 'core'
      overshoot_bdy_loc(1) = 'top'
      overshoot_f(1) = 0.335   ! step part, slightly less than brott
      overshoot_f2(1) = 0.01  ! exponential part, replace outer part of brott with this
      overshoot_f0(1) = 0.01