Add a function to reset the levels in a categorical column to the ones that are currently present
This function would reset the levels in a H2OFrame column. The use-case is that someone subsets a frame using a categorical column, but the new/partitioned H2OFrame would still return the original levels when queried. There are many use-cases where you'd want to keep the original domain, but also use-cases where you'd want to reset it with the subset of levels that exist in the new frame.
Proposed name in R: `h2o.reset_levels()`, and in python: `h2o.H2OFrame.reset_levels()`
I'm assuming that we don’t want to use the existing `h2o.setLevels(x, levels, in.place = TRUE)` function (where the levels must be same size as previous levels). in python its `h2o.H2OFrame.set_levels()`
Side note: Even though we have `h2o.setLevels()` (camelCase in R), we have been making aliases for all these really old functions to use snake_case, as prescribed by the tidyverse Style Guide, which is the style guide we follow in H2O-R. I don't think it's worth creating the camelCase version of the `h2o.reset_levels()` function; its sufficient to have the snake case only. Also, while we're here, we could add the missing `h2o.set_levels()` alias for `h2o.setLevels()` in this ticket.