Check for 'NaN' condition in transport_model()

[Joaobazzo]

When there are NaN speed values in the transport_model() ouput and no other valid value, it's not possible to perform a mean average to remove the NaN . In such cases, the function could add the mean 19 km/h (since 19 kph is the speed of driving conditions of transit buses in urban areas according to the EMEP/EEA).