REQS-498 Overnight arithmetic average conversion

Overnight arithmetic average conversion, better error logging for missing fixing data
The method "getOISDefinition" actually create a coupon overnight with arithmetic average. The method name could be changed to reflect that: getONArithmeticAverageDefinition (or something similar). 

In line 409, the error message "throw new OpenGammaRuntimeException("Unsupported OIS geared coupon");" could also the adapted to the "ArithmeticAverage" coupon.