Update aztarna/cmd.py

[LanderU]

In order to avoid:

lander@MacBook-Pro-de-Lander:~/github/aztarna (master)$ aztarna -t ros -p 11311 -i ros_scan_s20.csv -o lander2.csv
Exception occurred during execution
Traceback (most recent call last):
  File "/usr/local/bin/aztarna", line 11, in <module>
    load_entry_point('aztarna==1.0', 'console_scripts', 'aztarna')()
  File "/usr/local/lib/python3.6/site-packages/aztarna-1.0-py3.6.egg/aztarna/cmd.py", line 64, in main
    prompt = PROMPT
  File "/usr/local/lib/python3.6/site-packages/aztarna-1.0-py3.6.egg/aztarna/cmd.py", line 35, in main
    If the value of `self.intro' is nonempty when the cmdloop method is called,
  File "/usr/local/lib/python3.6/site-packages/aztarna-1.0-py3.6.egg/aztarna/commons.py", line 32, in load_from_file
FileNotFoundError: [Errno 2] No such file or directory: 'ros_scan_s20.csv'

Behavior after change:

lander@MacBook-Pro-de-Lander:~/github/aztarna (fix-FileNotFoundError)$ aztarna -t ros -p 11311 -i ros_scan_s20.csv -o lander.csv
Input file not found

[XabierPB]

Thanks!