Handle missing data points

[chris-reeves]

Handle missing data points when calculating thresholds. These are most likely to occur when attempting to measure UDP upload in areas with weak signal and the user chooses not to retry a failed measurment.

Without this, wifi-heatmap-thresholds fails with the following error:

Traceback (most recent call last):                                                                                                                                                                                                              
  File "/usr/local/bin/wifi-heatmap-thresholds", line 11, in <module>                                                                                                                                                                                                                                                          
    load_entry_point('wifi-survey-heatmap', 'console_scripts', 'wifi-heatmap-thresholds')()                                                                                                                                                                                                                                    
  File "/app/wifi_survey_heatmap/thresholds.py", line 127, in main                                                                                                                                                                                                                                                             
    ThresholdGenerator().generate(args.TITLE)                                                                                                                                                                                                                                               
  File "/app/wifi_survey_heatmap/thresholds.py", line 59, in generate                                                                                                                                                                                                                                                          
    min(x[key]) for x in items                                                                                                                                                                                                  
  File "/app/wifi_survey_heatmap/thresholds.py", line 59, in <listcomp>                                                                                                                                                                                                                                                        
    min(x[key]) for x in items                                                                                                                                                                                                  
TypeError: '<' not supported between instances of 'NoneType' and 'float'                                                                                                                                                                                                                                                       

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[jantman]

Apologies for letting this sit for so long, but thank you so much!