Mimic camchem substep convergence failure integration acceptance

[K20shores]

If the Backward Euler solver fails to converge, the state at the end of the last successful solver time step should be returned.

Acceptance Criteria

• When the Backward Euler solver fails to converge, the state at the end of the last successful solver time step is returned.

Ideas

• The Robertson analytical example can be used to test this. As of now, backward euler fails to converge for the robertson example
• This block of code could be modified to set state.variables_ = Yn prior to throwing then error:
  

  micm/include/micm/solver/backward_euler.inl

  Lines 171 to 179 in b819e57

  	if (n_convergence_failures >= time_step_reductions.size())
  	{
  	// we have failed to converge, accept the solution
  	// TODO: continue on with the current solution to get the full solution
  	n_convergence_failures = 0;
  	// give_up = true;
  	t += H;
  	throw std::system_error(make_error_code(MicmBackwardEulerErrc::FailedToConverge), "Failed to converge");
  	};