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