Outlier price filtering

[ghost]

Sometimes an exchange may report a stale price, esp. on thinly traded markets. This PR filters out prices too far away from the consensus average. Standard deviation of 2.2 chosen to determine outliers, configurable in application.properties.

From experimentation:

• Higher standard deviation (e.g. 5) results in no prices removed.
• Lower standard deviation (e.g. 1.5) results in too many prices removed.

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Current results:

 INFO  b.p.s.ExchangeRateController: Incoming /getAllMarketPrices request
 WARN  b.p.s.ExchangeRateService: outlier price removed=16700.0, source=QUOINE, ccy=EUR, consensus price=25651.2922 
 WARN  b.p.s.ExchangeRateService: outlier price removed=0.00172711, source=BTCMARKETS, ccy=LTC, consensus price=0.003372445 
 WARN  b.p.s.ExchangeRateService: outlier price removed=16200.0, source=QUOINE, ccy=USD, consensus price=27619.204166666666 
 WARN  b.p.s.ExchangeRateService: outlier price removed=0.07346, source=QUOINE, ccy=ETH, consensus price=0.0631171025