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: https://github.com/suyashkumar/dicom/blob/f77ffdec9a83ffddf27d05aeaeb97f7fb220e589/element.go#L66

[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)