Feature : Error based NL time step estimate

• Add a routine to estimate the NL source error based on order comparison

• Use the order based error check to enable timestep reduction

  Far from ideal as can currently "fight" with the traditional CFL limit checks.

• Add missing declaration

• Updating NL error estimate to use variable time step coefficients

• Only check the order based error method after specified number of steps

• Add some comments and use the order based target timestep to set the timestep limit and reset state

• Make some settings for order based error method be user inputs

Currently uses an approximation of the relative error.

Possible modifications include using a time history of errors to smooth out time step changes (i.e. to avoid regular increases and decreases in step size) and using a more robust error measure.