Changed Results schema according to discussion

* Changed the names of two fields (suggested in #29):
  * AggregationInterval (or AggregationLength) would replace
AggregationDuration, to acknowledge that this concept is equally valid
for spatial coverages as it is for time series.
  * AggregationStatisticCV would replace InterpolationTypeCV
* Added AggregationIntervalUnitsID to the ResultValues table (suggested
in #29).
* Replaced CoverageOrigins with ODM2SamplingFeatures.SpatialReferences
as suggested in issue #30.