Should return specific error if only a single rater provided as input instead of runtime error

[grigsos]

Krippendorff's alpha is a measure of inter-rater reliability, so there should be a check if it is a single rater instead. Here is where it currently fails

krippendorff\krippendorff.py:297: RuntimeWarning: invalid value encountered in scalar divide
  return 1 - (o * d).sum() / (e * d).sum()

when called by

krippendorff.alpha(reliability_data=newlistconvert,level_of_measurement="nominal")

[bryant1410]

Hey, I just committed to the main branch a check on this.