Patch release to address requests from editorial team at Journal of Statistical Software.

• Added note in the documentation for estimate_boot_sim_cv() that this function's output does not have a plot method. The closely-related function estimate_boot_reps_for_target_cv() does however have a plot method. For #54.

• Simplified structure of objects created by estimate_boot_reps_for_target_cv(), by removing the function component of output. For #54.