Improve mtz export

Now reduced data exported to mtz can be read by Aimless and other programmes in the CCP4 suite.

The MtzExporter class has been considerably simplified, and corrected to use the relevant library functions for writing mtz files, rather than writing directly to the mtz data structure. The batch data structures are now correctly generated (a batch is an image in the context of CCP4), and contain metadata on the instrument state (rotation matrices, axes and rotation angles).

Resolves #563 (closed), #567 (closed), #609 (closed)