Added include guards
Removed initial time from constructor arguments
Removed initial time t_init from constructor arguments; t initializes to zero
Defined write_state()
Don't write to stdout if writing to output file
Get input filename and (optional) output filename from arguments; get integration duration, initial time step and desired accuracy from input file
Added line for integration duration, initial time step and desired accuracy
Declaration for print_usage()
Updated with usage and input parameter file layout