Implement index management

This is not a viable solution:

DOCUMENT_CATEGORY_TO_INDICES_MAP = {
    'a': ['a_1', 'a_2'],
    'b': ['b'],
    'c': ['c_1', 'c_2'],
}

Indices & data types should not be hard-coded. We need additional api object for managing indices & their meta. Copy from TTK.