Pydicom-seg

- Dropped support for Python 3.6
- Replaced attrs with Python 3.7 dataclasses
- Fixed pixel spacing order when writing (similar to 37)
- Added support for loading a template from a pathlib.Path
- Added support for loading DICOMs from paths when writing a segmentation

- Use ContentCreatorName from template (27)
- Clear segment_buffer for every segment in SegmentReader (35)
- Fix/template code sequences (36)
- Fix pixel spacing conversion to SimpleITK (37)
- Fixed rounding error for calculating number of slices in both readers (43)
- First draft of speedup adding frames (44)

- Update pydicom requirement to allow 2.0 usage
- Fix package `__version__`

Allow SimpleITK version 2 or newer

Bug Fixes
- Add default preamble to segmentation dataset (25)
- Fix typo in documentation (23)

Features
- Added documentation (11)
- Added Github Actions for CI/CD (19)
- Refactored segmentation dataset/writer (9)

Bug Fixes
- Generated DICOM-SEGs were not standard conformant (21)
- Template import from single-file input metainfo.json (17)
- Wheel on PyPI was missing required external files from dcmqi (15)

Notes
- Removed Python 3.5 support due to usage of literal string interpolation (PEP 498)