Update docstring for viz, learn, fetch

For issue #77 (closed) Updated the docstring for

viz to

""" Visualize the results of the learn command
    
Serve HTML5 visualization from directory with
GRAPH_FILE (JSON data file with graph information
about nodes and edges)
"""

learn to

""" Analyze telemetry data
    
Apply machine learning and feature engineering
to analyse data from INPUT_FILE (path to input json file)
"""

fetch to

""" Obtain telemetry data
    
Retrieve and decode the telemetry corresponding to
SAT (satellite name or NORAD ID) and stores in
OUTPUT_FILE (path to the output folder)
"""

to make it more informative and relevant.

Signed-off-by: Adithya Venkateswaran adithyav1511@gmail.com