Bug fixes for private tags

[wetzelj]

Description

Related issues: #146 , #147

Please include a summary of the change(s) and if relevant, any related issues above.

• Modified the default VR for newly added tags from UN (Unknown) to ST (Short Text). (#146)
• Replicated replace identifiers unit tests to ensure that modified headers can be appropriately written as files. (#146)
• Modified expand_field_expression to utilize the tag number instead of the element keyword when processing %fields lists. (#147)

Checklist

• [x] I have commented my code, particularly in hard-to-understand areas
• [x] My changes generate no new warnings
• [x] My code follows the style guidelines of this project

Open questions

Questions that require more discussion or to be addressed in future development:

[wetzelj]

Thanks!

[vsoch]

https://pypi.org/project/deid/0.2.19/