recordTable: n_images column

[jniedballa]

Check effect of removeDuplicateRecords on n_images column, document what the column shows (in vignette).

Reprroducible example: examples(recordTable)

See difference between rec_table3a and rec_table3b.

Edit: n_images is wrong when removeDuplicateRecords = F and minDeltaTime = 0.