step_values used index_at_value with unrecognised endpoint argument. Arguments removed as ineffective.
index_at_value assertion added to check that endpoint argument is recognised. Additional option for first point when closing to steady state added. (Not currently used).
step_values: scan_fwd start rounded down to include first sample of transition array. Test specifically to confirm this solves a problem seen on the 767-4 frame added, although other step_values tests now fail.
step_values: test plots commented out.