Refactor to create frame, element, write, and query packages

[suyashkumar]

This change refactors the element and frame entities into their own packages. This does cause some annoying stutter (for example frame.Frame as opposed to dicom.Frame) which is something I am going to think about a bit before merging. There are many examples in the std lib of this, though (like context.Context for example.

This change also factors write and query logic into their own packages.

[suyashkumar]

Also, I do not really like element.DataSet ... perhaps DataSet should get its own package at some point, or the element package should get a different name.

[suyashkumar]

we should consider putting many of these packages into pkg/. However I do like having the main API exposed at the top level (can just import and use github.com/suyashkumar/dicom)...