Updated pydicom dependency from 1.3.0 to 2.1.1

[wetzelj]

Description

Related issues: #171 Updated required pydicom version from 1.3.0 to 2.1.1

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

None