This change add the sigavg function from the original WFDB package. So far, I have included the -d, -f, -t, and -v flags with intentions to add the flags -p (done in 7ed6c24) and -z before merging. For this implementation I decided to allow the user to both print the output like in the original package and return it as a Pandas dataframe for easy lookup and access when piping it to another function.