get_next_image: Find the best-next image for a fusion model.
In albhasan/sits.starfm: Use the StarFM Fusion Model on Landsat 8 Images

Find the next best image from the given reference row in the brikcs' images. In case of no match, the next image with the fewer clouds is returned.

1	get_next_image(brick_imgs, ref_row_number, cloud_threshold = 0.1)

`brick_imgs`	A tibble of the images in a brick, including the variables img_date, sat_image, and cloud_cov.
`ref_row_number`	A length-one numeric. The index of the reference row in brick_imgs.
`cloud_threshold`	A length-one numeric. The threshold for matching such that the product of the cloud coverages of the reference and the match are less than this threshold.