Consider updating type system to use Go generics

[suyashkumar]

Now that go has generics we will want to update the Element value APIs to make use of them at some point:

dicom/element.go

Line 66 in f77ffde

type Value interface {

[webstradev]

@suyashkumar Are you just referring to using any instead of interface{} wherever possible, if so I'd be happy to go through the repository (either in batches or all together, whatever you prefer)