Write a method that returns mfcc and gfcc features for an input audio file

[jsingh811]

Currently, mfcc and gfcc features are being computed as a mid step of a classification problem and we don't get a direct view of the features.
One can want to look at these features independently and use them with a different classification backend.

• implement a function to return mfcc for an input audio file
• implement a function to return gfcc for an input audio file
• update documentation to guide the usage of these new functions
  Thus, we want to write a function that outputs mfcc and a function that outputs gfcc feature for an input audio file.