* Added code to serialize VolumeDataLayout objects
* Added test for serialization of VolumeDataLayout objects * Added a 'keys' vector to the MetadataContainer and added a MetadataType field to the keys to allow for ordered iteration through the metadata key/value pairs. * Changed the AWS settings for test data to be CMake options instead of environment variables to make it work the same as locating test data. * Added a getLayoutDescriptor() method to VolumeDataLayout and added some member variables to hold missing information about overall LOD settings. * Fixed Base64Encode() functions and allow empty input to Base64Decode() function.