Add functionality for modifying and creating data

[mbollmann]

This PR develops still missing functionality for modifying existing data as well as ingesting new proceedings.

When done, will close #4338.

[codecov]

Codecov Report

Attention: Patch coverage is 93.62187% with 28 lines in your changes missing coverage. Please review.

Project coverage is 92.32%. Comparing base (dfda10f) to head (9905769).
Report is 1 commits behind head on python-dev.

Files with missing lines	Patch %	Lines
python/acl_anthology/collections/paper.py	91.91%	8 Missing ⚠️
python/acl_anthology/utils/attrs.py	88.52%	7 Missing ⚠️
python/acl_anthology/collections/bibkeys.py	93.33%	4 Missing ⚠️
python/acl_anthology/collections/collection.py	89.65%	3 Missing ⚠️
python/acl_anthology/collections/volume.py	91.17%	3 Missing ⚠️
python/acl_anthology/collections/event.py	97.36%	1 Missing ⚠️
python/acl_anthology/collections/index.py	94.44%	1 Missing ⚠️
python/acl_anthology/text/stopwords.py	95.45%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff               @@
##           python-dev    #4373      +/-   ##
==============================================
- Coverage       92.56%   92.32%   -0.25%     
==============================================
  Files              32       35       +3     
  Lines            2301     2566     +265     
==============================================
+ Hits             2130     2369     +239     
- Misses            171      197      +26     

Files with missing lines	Coverage Δ
python/acl_anthology/anthology.py	89.40% <100.00%> (+0.36%)	⬆️
python/acl_anthology/collections/__init__.py	100.00% <100.00%> (ø)
python/acl_anthology/collections/eventindex.py	96.00% <100.00%> (ø)
python/acl_anthology/collections/types.py	100.00% <100.00%> (ø)
python/acl_anthology/constants.py	100.00% <100.00%> (ø)
python/acl_anthology/containers.py	96.26% <100.00%> (ø)
python/acl_anthology/exceptions.py	96.00% <ø> (ø)
python/acl_anthology/files.py	98.78% <100.00%> (ø)
python/acl_anthology/people/index.py	83.73% <100.00%> (ø)
python/acl_anthology/people/name.py	98.55% <100.00%> (+0.02%)	⬆️
... and 18 more