Have bG rating numbers for the day/week/month

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Mean bG based on area-under-the-graph - hence time-weighted.
A normalised figure for the sum of the difference between readings.

• Eg a bad day with these readings: 3,10,5,12,16,9,3 => sum(abs(n-m) = 33
• Eg a good day with these readings: 4,7,5,7,6,5,6 => = 10
• Count normalised: sigma * num / mean count
• A small ascent/decent implies stability

Penalty flags for hypo/hyper should exist (#15)

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Mean bG area under graph done.