calculate average using system_time instead of assuming 1 data point every 5m

[scottleibrand]

Starting at https://github.com/openaps/openaps-js/blob/master/bin/determine-basal.js#L8 we determine the average delta over the last 3, 2, or 1 available CGM data points. In cases where CGM data is missing, this results in an overestimate of the (absolute value of the) average delta, which can cause OpenAPS to overreact until it gets more CGM data.

It would be better to calculate the average delta using the elapsed CGM system time between the two data points under comparison, instead of naively assuming each data point represents 5 minutes.