Add select_ltypes to DataTables to allow user to select Data Columns

[gsheni]

• Similar to how pandas has select_dtypes function, there should be a function on DataTable that allows users to select_logical_types or select_ltypes.
• User can pass in a list of Logical Types, a list of strings (class name or camelCase type string) of Logical Types, or 1 specific Logical Type objects or a list of Logical Type objects
• In the future, we can look into adding exclude to select_ltypes

df = pd.read_csv(...)
dt = DataTable(df, name='data')
# support single string
dt.select_ltypes('bool')
# support type name as string
dt.select_ltypes('zip_code')

# support list of type_string of Logical Type
dt.select_ltypes(['categorical', 'natural_language'])
# support list of string of class name
dt.select_ltypes(['Categorical', 'NaturalLanguage'])

from data_table.logical import Categorical, NaturalLanguage
# support actual Logical Types objects
dt.select_ltypes(Categorical)
# support actual Logical Types list of objects
dt.select_ltypes([Categorical, NaturalLanguage])

[tamargrey]

Some implementation questions

• Is the selection happening from the original dataframe always or whatever the current state of the DataTable is?
  • My guess would be we always apply to the DataTable's columns, so if we've already changed some aspects of the logical or semantic types, those changes get propagated.
• Can input lists be of mixed types - so something like ['boolean', Categorical]?
• Do we want to be case blind in logical type strings?
  • Just thinking that we do this with primitives in dfs
• Should the underlying dataframe also change when we select certain DataTable columns?
  • I might expect the underlying df to never change, but I haven't seen that explicitly stated anywhere
• Do we want any warnings if no columns fall under any of the ltypes specified (empty DataTable) or if all of them apply (no change from the original)?
• Should this remove the index and time_index columns even if their ltypes aren't included?

[gsheni]

@tamargrey

• The selection is happening ont he current state of the DataTable. We would always apply it to the DataColumns on the DataTable.
• Yes, the input lists can be mixed types.
• Yes, let's be case blind (upper/lowercase). If we are doing that with primitives and DFS, let's do that in this case.
• The underlying dataframe should not change. This is a helper function to return DataTable based on the inputted Logical Types.
• If no logical types fall under the specified, let's return an empty DataTable with an empty dataframe.
  • If all logical types fall under the specified, return the full DataTable with all columns.
• Yes, for now, we can remove the index and time_index if the logical types are not included. We can revisit this behavior in the future. But for now let's be specific.