Small cell fixes

[phyy-nx]

• For small cell, better handle symmetry operators
• For small cell, don't overwrite imageset_id if present

Required for dials/dials#2477

[phyy-nx]

@dwpaley I also checked in here the defaults we've been using lately.

[bkpoon]

/azp run Checks

[azure-pipelines[bot]]

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