ann2rr returns ghost beats

[OliverFaust]

Hi,

If the input file, say `atr', contains additional labels, such as (N and (AFIB, ann2rr returns surplus rr intervals at the location of these labels.

Details:
In ann2rr in hr.py the rr intervals are calculated:
rr_interval = calc_rr(ann.sample, fs=ann.fs)

ann.sample contains the beat locations + the annotation location. Hence, ann2rr splits the rr interval which spans over each annotation location into two. This changes the heart rhythm at critical points. For example, at the beginning and end of atrial fibrillation, i.e. at (AFIB and (N.