make Daru::DataFrame#covariance faster (#345)
make Daru::DataFrame#covariance faster (#345)

* make Daru::DataFrame#covariance faster

* avoid offending rubocop rule

rohitner/daru,SciRuby/daru,v0dro/daru,rohitner/daru,v0dro/daru,SciRuby/daru