function isUnitForSubtotals(sName) {
				return Object.keys(oAggregation.aggregate).some(function (sAlias) {
					var oDetails = oAggregation.aggregate[sAlias];

					return oDetails.subtotals && sName === oDetails.unit;
				});
			}
Returns the "$orderby" system query option filtered in such a way that only aggregatable
or groupable properties, units, or additionally requested properties are used, provided
they are contained in the given aggregation information and are relevant for the given
level. Items which cannot be parsed (that is, everything more complicated than a simple
path followed by an optional "asc"/"desc") are not filtered out.

@param {string} [sOrderby]
  The original "$orderby" system query option
@param {object} oAggregation
  An object holding the information needed for data aggregation;
  (see {@link .buildApply})
@param {number} [iLevel=0]
  The current level; use <code>0</code> to bypass group levels
@returns {string|undefined}
  The filtered "$orderby" system query option or <code>undefined</code>

@private