Do not skip subject if other contrast does not exist

This PR:

Add a condition to check if other contrast (specified by the -load-other-contrast flag) exists. If the other contrast does not exist, only the main contrast (specified in the config file provided by the -config flag) is open - https://github.com/spinalcordtoolbox/manual-correction/commit/675a894bf45c6ad7a87dee9aa6aaa2161ca1b10e, https://github.com/spinalcordtoolbox/manual-correction/commit/a69d30e92d98a68e84ffd6ddc418a749cdb9c921
Refactor the correct_segmentation() function to make it shorter and easier to follow - https://github.com/spinalcordtoolbox/manual-correction/commit/362ee0607dcf683788f512a4c434048e17a1b168

spinalcordtoolbox / manual-correction