title: HealthKit: Reading HKCorrelationType is not allowed
body: <p>When I attempt to initialize HealthKit with an HKCorrelation sample type, the app crashes with 'Authorization to read the following types is disallowed: HKCorrelationTypeIdentifierBloodPressure'.</p>
  
  <p>I've successfully read from a variety of quantity types and the sleep category types. </p>
  
  <p>The code is not contiguous but I'm calling </p>
  
  <pre><code>[healthStore requestAuthorizationToShareTypes:writeDataTypes
                                      readTypes:readDataTypes
                                     completion:^(BOOL success, NSError *error) {
  ... 
  }
  </code></pre>
  
  <p>where readDataTypes is an NSSet containing the set of sample types I am looking to read. One of them is HKCorrelationTypeIdentifierBloodPressure. </p>
  
  <p>When I remove the blood pressure key from the set it works fine.</p>
  
  <p>The set also includes HKQuantityTypeIdentifierBloodPressureSystolic and HKQuantityTypeIdentifierBloodPressureDiastolic sample types.</p>
  
  <p>Does Apple not want us reading the combined data type?</p>