Add costs fields to Worker model, admin and serializer

arkindex/documents/fixtures/data.json

arkindex/documents/management/commands/build_fixtures.py

arkindex/process/admin.py

arkindex/process/migrations/0048_worker_cost_fields.py

arkindex/process/models.py

arkindex/process/serializers/workers.py

arkindex/process/tests/test_corpus_worker_runs.py

arkindex/process/tests/test_corpus_worker_versions.py

arkindex/process/tests/test_user_workerruns.py

arkindex/process/tests/worker_runs/test_create.py

arkindex/process/tests/worker_runs/test_list.py

arkindex/process/tests/worker_runs/test_partial_update.py

arkindex/process/tests/worker_runs/test_retrieve.py

arkindex/process/tests/worker_runs/test_update.py

arkindex/process/tests/worker_versions/test_create.py

arkindex/process/tests/worker_versions/test_list.py

.../tests/worker_versions/test_recommended_worker_version.py

arkindex/process/tests/worker_versions/test_retrieve.py

...ex/process/tests/worker_versions/test_retrieve_feature.py

arkindex/process/tests/workers/test_create.py