set ts directly in ProblemDefinition.update_time_stepper()
new AdaptiveTimeSteppingSolver - implicit adaptive time sptepping solver

- new get_min_dt(), adapt_time_step()
- new VariableTimeStepper.from_conf(), .set_from_ts(),
  .set_n_digit_from_min_dt()
- update VariableTimeStepper.set_step() to allow only step = 0
- update VariableTimeStepper.__iter__() to include t1
update linear_elastic_damping.py example to use adaptive time stepping