Added implementation and test for multiple mainfiles in one directory.

nomad/api/repo.py

nomad/files.py

nomad/processing/data.py

tests/conftest.py

tests/data/proc/examples_template/1.aux

tests/data/proc/examples_template/2.aux

tests/data/proc/examples_template/3.aux

tests/data/proc/examples_template/4.aux

tests/data/proc/examples_template/template.json

tests/processing/test_data.py

tests/test_api.py

tests/test_files.py

tests/test_normalizing.py