fix UT

del unused arguments

fix UT

explainer_api UT change to pytest style

fix UT url

explainer encapsulator UT change to pytest style

remove unnecessary deepcopy

remove import copy

fix some coding style issues

json return null if ExplainManager returns None

use fromtimestamp

use fromtimestamp

wrong name

wrong json key

use offset as page offset

enhance comments

rm mock_explainer_api.py

update copyright year 2020

remove unused import