identify numeric columns

[victor-luu191]

Describe the new feature:

This is useful to gather all numeric columns into a list so that later we can apply any transformation on them in one-liner style. For example, fill NAs in the numeric columns by their means.

What is the current outcome?

Module nominal already has method identify_nominal_columns to find nominal columns. I follow its style and added to my fork a method identify_numeric_columns, but it seems I cannot make a pull request yet because there is another request pending. Can @shakedzy accept my previous pull request?

Is it backward-compatible?

Should be no problem, as it is independent from others.

[shakedzy]

Nice idea, both this one and the missing values. I've add a new method, identify_columns_by_type which is more general, and now identify_nominal_columns and identify_numeric_columns are shortcuts to it.