Re-engineer goto continue statements

[aosprey]

Identified in #27, but the goto continues are a bit more complicated to remove. In particular in mckpp_physics_ocnstep, it would require restructuring the WHILE loop:

DO WHILE flags
  stuff

  IF (L_KPP) 
45  CONTINUE
    stuff

    IF (cond) 
      GOTO 45
    END IF

    stuff
  END IF
  
END DO