average delta calculation

[dm61]

Currently, avg_delta in glucose-get-last.js by taking the current bg, subtracting a past bg value, up to 3 steps behind (15 min old), and dividing by the minutes between. A problem with this approach is that avg_delta is about as noisy as delta. Any decisions based on avg_delta are bound to be corrupted by noise. A better approach is to use an FIR filter over past n samples (this could be a variable) to estimate an average slope. Here is an example that compares delta FIR filtered avg_delta using the current bg and past 6 values, so averaging is over 30 minutes. This FIR filter is equivalent to finding the slope of the best-fit linear-regression line through the bg points. Should investigate other FIR filters.