[Enhancement] Support get sub dataset and rename method and variable. (#145)

* add get_subset method, add comment, rename variable

* add unit test

* Please mypy

* Fix as comment, support negative index, and fix index access error

* add and refine docstring, handle indices=0

* handle indices=0

* add empty list indices test

* rename in_meta in docstring and comments to in_metainfo

* clean meta nameing

* Fix negative indices error

* test empty list of get_subset

* fix comments and docstring

* add unit test

* Fix as comment

* Fix as comment

* add docstring to mention wrapped dataset should not inherit from Basedataset

* Fix wrapped dataset docstring

* Fix wrapped dataset docstring

* Fix method name, docstring, and comments

* Fix comments

* Fix comments

* Fix comments