Fix/remove sequences

[wetzelj]

Description

Related issues: #137 Added additional check in tags.py to ensure that the element exists prior to attempting the element deletion in the remove_sequences function. Nested sequences will be removed when the parent sequence is removed, and therefore, when the child is attempted to be removed (as a result of being in the initial dicom.iterall()), it will not be present, previously resulting in a KeyError. This fix, checks to ensure that the sequence being deleted still exists in the header prior to attempting the deletion.

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: To keep this PR tightly scoped, this is limited to just the strip_sequences functionality with replace_identifiers. This is not intended to evaluate or test specific actions against tags that exist as top-level tags and child tags within a sequence.

[vsoch]

Looks great! Sorry for the delayed response, GitHub pages has been coughing and I've been trying to release the RSE Stories podcast episode - it finally went through. I've already reviewed this and you added a great docstring to explain the test, so I'm good to merge!