Added the find section by code functionality

Description

I have updated the cdaannotator by adding the _find_section_by_code() function and modifying the related functions such that if the cda document doesn't contain template_ids then it can identify and retrieve the related sections using their respective code values .

Changes Made

-Added SectionCode Enum. -Added _find_section_by_code() function. -Updated the related functions like _find_problems_section(). -Added and updated tests for a document which doesn't contain template_ids for various sections .

Testing

I have added tests similar to the already existing tests but with a different instance of the cdaAnnotator which contains data from that xml document which doesnt have any template_ids.

Checklist

[x] I have read the contributing guidelines
[x] I have performed a self-review of my own code
[x] I have commented my code, particularly in hard-to-understand areas
[ ] I have made corresponding changes to the documentation
[x] My changes generate no new warnings
[x] I have added tests that prove my fix is effective or that my feature works
[x] New and existing unit tests pass locally with my changes

dotimplement / HealthChain