Handle shared models in dials.index with known orientations

[jbeilstenedmands]

Fixes #1819

[jbeilstenedmands]

Just to demonstrate the output on some of the l-cysteine test data:
Command
xia2 image=/Users/whi10850/dials/build/dials_data/l_cysteine_dials_output/l-cyst_01_00001.cbf:1:15 image=/Users/whi10850/dials/build/dials_data/l_cysteine_dials_output/l-cyst_02_00001.cbf:1:15 min_spots.per_degree=10 xtriage_analysis=False multi_sweep_indexing=True
Followed by
dials.index 9_indexed.{expt,refl}
This PR:

+------------+-------------+---------------+-------------+
|   Imageset |   # indexed |   # unindexed | % indexed   |
|------------+-------------+---------------+-------------|
|          0 |          42 |             7 | 85.7%       |
|          1 |          30 |             3 | 90.9%       |
+------------+-------------+---------------+-------------+

dials-main:

+------------+-------------+---------------+-------------+
|   Imageset |   # indexed |   # unindexed | % indexed   |
|------------+-------------+---------------+-------------|
|          0 |          42 |             7 | 85.7%       |
|          1 |           0 |            33 | 0.0%        |
+------------+-------------+---------------+-------------+