- use GAT on Element type
- add element_opt and element_by_name_opt

- 1-level deep copy of a DICOM value

- add DicomAttributeValue trait,
  use it as output of most methods in DicomObject
- rename methods and add new ones in DicomObject
   - use GATs for the attribute value, item and pixel data types
- reimplement DicomObject

- provide access to items and pixel data fragments
- require DicomAttributeValue to impl DicomValueType
- add a few tests

- rename from DicomAttributeValue
- add `to_i32` and `to_u16`

- instead, meta attributes can be retrieved
  like any other attribute

- replace `to_dicom_value` with `to_primitive_value`
   - simplifies implementations
   and requires consumers to
   depend on other methods
   when working with sequences
- remove method `meta`
   - treat meta information attributes like any other attribute,
   retrievable through the same methods
- [core] add either crate
- impl many DICOM traits to `either::Either`
- implement DicomObject for FileMetaTable
- reimplement DICOM traits for FileDicomObject
  so that users can retrieve
  either meta info or main data set info

- elide lifetime
- fix formatting of doc comments