Issue 183 : Enable reader group Python APIs

[shrids]

Change log description

• Enable Reader group Python APIs

Purpose of the change
Fixes #183

What the code does
This change enables the ReaderGroup APIs for the python client.
Example snippet is shown below.

import pravega_client;
import asyncio;

stream_manager = pravega_client.StreamManager("127.0.0.1:9090")
reader_group = stream_manager.create_reader_group("rg-1", "scope", "testStream")
r1 = reader_group.create_reader("reader-1")
r2 = reader_group.create_reader("reader-2")
slice= await r1.get_segment_slice_async()
for event in segment_slice:
    print(event.data())
# release a segment slice back. Releasing a partially consumed segment slice
# ensures the next segment slice has the remaining data.
r1.release_segment(slice)
# mark the reader as offline.
r1.reader_offline()

How to verify it
Python tests have been implemented. This can be invoked using the tox tool.[https://tox.readthedocs.io/en/latest/]

[codecov-io]

Codecov Report

Merging #182 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #182   +/-   ##
=======================================
  Coverage   74.39%   74.39%           
=======================================
  Files          51       51           
  Lines        9416     9416           
=======================================
  Hits         7005     7005           
  Misses       2411     2411           

Impacted Files	Coverage Δ
src/reader_group_config.rs	84.31% <100.00%> (ø)