cross validation error for rows with 0s

[jeffwong]

cross validation division by 0 error

[jeffwong]

cv now reports rmse and percent rmse.

Percent rmse is misleading - if there are two missing values that need to be imputed

actual = 100,2 estimated = 90,1

Percent rmse = sqrt(0.5(10%^2 + 50%^2)) = 36% rmse = sqrt(0.5(10^2 + 1^2)) = 7.1

Note that the 50% error on the second value plays a much smaller role in the rmse