Reduce the number of dependencies

[mpizenberg]

• Input-Output (IO) should be independent from this library. This concerns the image, png, byteorder and nom crates.
• itertools is used only once for the izip function, to zip 3 vectors in the same loop. It probably isn't worth the dependency.
• num-traits is only used in dso candidates selection implementation for the Number trait. Maybe there is a way of doing it without the dependency. Maybe it shouldn't even be generic since for now, this implementation is only done for u16 gradient images.
• Random number generation is only used in candidates::dso. DSO is also the cause of num-traits, and one implementation should not bring too many dependencies. It might be a good idea to make candidates selection a trait somehow, and separate coarse to fine and dso's implementations into another code base?