Refs #6428 Added method to DiskBuffer to set free space blocks (for loading). In getDiskBufferFileData() the free space blocks information is now set after reading the free_space_blocks group from the NeXus file.
Refs #6428 Added unit tests for DiskBuffer.
Refs #6428 Fix for test failure  - if the free space block vector only contains an empty entry this should not be added to the map.
Refs #6428 Added unit test for saving out and reading back the free space vector.
#6428 Removed logging intended just for debugging purposes.
Refs #6428 Removed unnecessary read only check. Could prevent issues where a file is changed from read only to not writable.
Refs #6428 Replaced hardcoded string with appropriate variable.
Merge remote-tracking branch 'origin/feature/6428_Load_algorithm_should_restore_free_spaces'