add some analytics based on miniSASS data

[gubuntu]

Expressed by Mark Graham to Gavin Fleming in June at the SAGTA conference

With many years of observations including some at the same site representing a time series, it would be really useful to see some insights from the data, live on the miniSASS site

e.g.

• summary stats like period, countries, numbers of users and observations
• change over time at a site
• river indicators - aggregating observations to show latest river or reach health and whether it's improving or getting worse