Add call to sort_packages and method placeholder

[rubenrails]

Adding sort_packages (as in spree core's prioritizer) so developers don't need to override prioritized_packages in order to add that call.

cc @huoxito

[huoxito]

Out of curiosity have you found a use case for that method? I read this issue on spree-guides and couldn't think of a good use for it spree/spree-guides#144 In a way packages are already sorted in Adjuster that's why I didn't add the sort_packages method in AssemblyPriotirizer.

[rubenrails]

Actually we're doing something in that method to ensure we give priority to one stock location for some products. Problem is that if we don't do anything then the adjuster will create a package for the undesired location leaving the right location's package as quantity=0 for the parts of the assembly.

[huoxito]

That sounds ok to me @rubenrails thanks for pointing out. Do you think you could submit a PR with an example code to spree-guides? That would be great too. Thanks!

[rubenrails]

Will do! Thanks :)