Function to add categories to a categorical column

[kkraus14]

Right now there is no way to add categories to a categorical column without defining a new column. This would be useful for UDFs, fillna, etc. where a user may want to split a categorical column based on another column or fill in nulls.

Example:

import pandas as pd
from pygdf.dataframe import DataFrame

pdf = pd.DataFrame({"cat_key": ["a", "b", None, "c", None], "value": [1, 2, 3, 4, 5]})
pdf['cat_key'] = pdf['cat_key'].astype("category")

gdf = DataFrame.from_pandas(pdf)

In the above the only thing we can fillna to is "a", "b", or "c". There should be an add_categories function similar to pandas.Series.cat.add_categories.

[kkraus14]

This functionality now exists as of 0.14. Closing.