Spatian span memory step.

This step type is used to present the interactive spatial span memory activity. You are not supposed to use this step on its own. Use ORKOrderedTask’s +spatialSpanMemoryTaskWithIdentifier:intendedUseDescription:initialSpan:minimumSpan:maximumSpan:playSpeed:maximumTests:maximumConsecutiveFailures:customTargetImage:customTargetPluralName:requireReversal:options: method to get a complete spatial span memory activity task instead.