Preserve imageset_id information, and unindexed reflections, in dials.combine_experiments

[ndevenish]

Previously, the "imageset_id" column of the reflection table became meaningless after dials.combine_experiments because the mapping between imageset and reflection was lost for unindexed reflections. In addition, dials.image_viewer and dials.reciprocal_lattice_viewer relied on this mapping for indexed reflections, leading to #1093.

This change preserves the mapping by working out what the old and new indexes were, and rewrites the reflections. Because this mapping is preserved, unindexed reflections can now be carried over to the combined file.

I've also rewritten the test to use the l_cysteine_dials_output test output instead of vmxi_proteinase_k_sweeps, because the latter does not include unindexed reflections.

Partially #1093 fix. dials.scale probably also requires separate attention.