Reland "C++ porting of the initial python script for conversational speech generation."

webrtc/modules/audio_processing/test/py_conversational_speech/README.md

-#Conversational Speech generator tool
-
-Python tool to generate multiple-end audio tracks to simulate conversational
-speech with two or more participants.
-
-The input to the tool is a directory containing a number of audio tracks and
-a text file indicating how to time the sequence of speech turns (see the Example
-section).
-
-Since the timing of the speaking turns is specified by the user, the generated
-tracks may not be suitable for testing scenarios in which there is unpredictable
-network delay (e.g., end-to-end RTC assessment).
-
-Instead, the generated pairs can be used when the delay is constant (obviously
-including the case in which there is no delay).
-For instance, echo cancellation in the APM module can be evaluated using two-end
-audio tracks as input and reverse input.
-
-By indicating negative and positive time offsets, one can reproduce cross-talk
-and silence in the conversation.
-
-IMPORTANT: **the whole code has not been landed yet.**
-
-###Example
-
-For each end, there is a set of audio tracks, e.g., a1, a2 and a3 (speaker A)
-and b1, b2 (speaker B).
-The text file with the timing information may look like this:
-
-```
-A a1 0
-B b1 0
-A a2 100
-B b2 -200
-A a3 0
-A a4 0
-```
-
-The first column indicates the speaker name, the second contains the audio track
-file names, and the third the offsets (in milliseconds) used to concatenate the
-chunks.
-
-Assume that all the audio tracks in the example above are 1000 ms long.
-The tool will then generate two tracks (A and B) that look like this:
-
-**Track A**
-```
-  a1 (1000 ms)
-  silence (1100 ms)
-  a2 (1000 ms)
-  silence (800 ms)
-  a3 (1000 ms)
-  a4 (1000 ms)
-```
-
-**Track B**
-```
-  silence (1000 ms)
-  b1 (1000 ms)
-  silence (900 ms)
-  b2 (1000 ms)
-  silence (2000 ms)
-```
-
-The two tracks can be also visualized as follows (one characheter represents
-100 ms, "." is silence and "*" is speech).
-
-```
-t: 0         1        2        3        4        5        6 (s)
-A: **********...........**********........********************
-B: ..........**********.........**********....................
-                                ^ 200 ms cross-talk
-        100 ms silence ^
-```