We should add a warning for the python/r method h2o.cor(), that tells users the method is only intended for numeric columns, if they try to pass a categorical column.

we should also add a runit/pyunit test to test what happens if a user passes a categorical. Right now it seems that we return NA for categorical columns with more than two levels.

{code}
library(h2o)
h2o.init()

create a categorical column called k2 with 5 levels and 20 values

k2 = rep(c('her', 'him', 'cat', 'mouse', 'dog'),4)

create a categorical column with two levels

k = rep(c('her', 'him'),10)

#create a numeric column with 20 values
n <- 20
h <- runif(n)

see what happens if you try to calculate the correlation of a numeric with a binary categorical

h2o.cor(as.h2o(k),as.h2o(h))

0.07981525

see what happens when you try to calculate the correlation of a numeric with a multi-level categorical

h2o.cor(as.h2o(k2),as.h2o(h))

NA

{code}