Tracking issue for changes of fork

[hobofan]

Current branch with cumulative changes: https://github.com/field33/oxigraph/tree/support_duration_average

Individual branches with changes:
https://github.com/field33/oxigraph/tree/support_duration_average : Contains an implementation for AVG of xsd:duration that works as long as the xsd:yearMonthDuration portion has a value that is identical to 0. Support for xsd:yearMonthDuration has been excluded as there isn't a straightforward way to convert a xsd:yearMonthDuration to a xsd:dayTimeDuration for an indeterminate point in time (due to high variance of lengths of months). Would require some polishing and better edge case handling to upstream.