Refactored repeated code across the ETT and Custom Dataset classes

[sergiovaneg]

I noticed that there are many repeated methods across Dataset_ETT_hour/Dataset_ETT_minute and Dataset_Custom, so I did a quick refactor using inheritance and abstracting the differences as class methods. I also did some minor style fixes (removed redundant elif statements and split long lines).

A similar process can be applied to the Seg loaders, but I'd rather know if you're interested in merging this kind of contribution before going any further.