Added comment about header/data for each timestep

We would like to build up and write the header only once (with
ecl_sum_fwrite_smspec) and then the data for each timestep (with
ecl_sum_fwrite_step), but since the data attach to the summary
object and so does the header, how do we keep the header in memory
without also accumulating all the data?
Make starting timestep more explicitly known

One-based or zero-based? Better be documented (in a way that doesn't
promote hard-coding in the clients!)
Don't use the wrong index for the timestep
Write the field data for the initial timestep
fixup! Don't use the wrong index for the timestep
fixup! Write the field data for the initial timestep
Don't write latest timestep twice for skipped report

If the simulator skips a reporting timestep, the latest timestep
should not be written twice to compensate (!).