Fixed method get_elapsed_seconds() when using pause()
Removed pause testing from the main test file

Reason: the code bacame too complex.
Correct work of get_remaining_time() after resume()
Renamed `_paused` to `_is_paused`
Fixing a bug that it was not possible to spawn a paused object
Fixes in serialization test
Fixes in POD
Added tests to get 100% branch test coverage