improvement: call from terminal

[GildasLepennetier]

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Hi there. I am suggesting to add a sheebang to be able to call the function easily from the terminal #!/usr/bin/env python

Also, I think the 'node' option is not clear in the help. I am not even sure I understood exactly why it is required. Anyway, some improved help would be good:

print ("\nUsage: python json_to_csv.py <node> <json_in_file_path> <csv_out_file_path>\n\n\tnode: node name, prefix for columns names\n")