Add remaining S3 methods

[mmulvahill]

For pulse_spec:

• Finish existing print.pulse_spec() function

For pulse_sim:

• print.pulse_sim
  • Print summary of input parameters and pulse features, then first 5-10 rows of time-series dataset
• Skip the summary.pulse_sim if no calculated summary stats to return

For pulse_fit:

• print.pulse_fit
• summary.pulse_fit
  • with HPD's, medians, CI width option -- see other related issue
• plot.pulse_fit
  • what would this look like? The fitted estimate/CI?