Prevent Miller indices from changing once assigned

This merge request is to address the observation that some peaks in the unmerged output have identical Miller indices. This is technically possible even if the total rotation range is under 180 degrees; however, it would be better to also ensure that once Miller indices are set, they are never changed.

• Miller indices of predicted peak collections should be immutable.
• Difficult to make them literally immutable because found peaks are created without indices, and must be indexed later.
• Before, Miller indices were not stored as a member variable, but computed as needed. The rounding involved allows the possibility that the Miller indices of a peak will change after the instrument states and unit cell are refined. Now they are generated only once: i) after indexing ii) when peaks are predicted iii) when a peak is copied
• Added benefit of reducing computing overhead of refreshing peak tables

Resolves #406 (closed)