More documentation changes. Added initial API docs stubs

docs/source/api/hugin.core.rst

docs/source/api/hugin.io.rst

docs/source/api/hugin.postprocessing.rst

docs/source/api/hugin.preprocessing.rst

docs/source/api/hugin.rst

docs/source/api/hugin.scene.rst

docs/source/api/index.rst

docs/source/conf.py

docs/source/index.rst

src/hugin/io/dataset_loaders.py