RK4 solver fixes, improvements

o Bug fix in take_step: was using f0 rather than start
  so adaptive stepping error control broke

o Adaptive timestepping must now be switched on using adaptive=true
  By default now fixed timestep

o Timestep now set using "timestep = .." setting, rather than "start_timestep"
  to be consistent with karniadakis solver

o Updated timestepping solver manual entry