Write SLP files (#55)

* Add convenience attribute to HDF5Video to check for embedded images

* Do BGR -> RGB conversion in OpenCV backend

* Remove unused kwarg filter

* Add videos metadata writing for SLP files

* Add tracks writing

* Tweak HDF5 utils to create files if non-existent.

* Add symmetry conveniences and flipping helper.

* Add metadata and skeletons writing

* More symmetries test coverage

* Initial implementation of point/instance/frame writing

* Tests

* Fix structured array construction

* Add full labels writing

* Add high level API

* Update fixture to use relative instead of absolute path to video

* Add more docs on the revised fixture

sleap_io/__init__.py

@@ -17,6 +17,7 @@
 from sleap_io.model.labels import Labels
 from sleap_io.io.main import (
     load_slp,
+    save_slp,
     load_nwb,
     save_nwb,
     load_labelstudio,

sleap_io/io/main.py

@@ -19,6 +19,16 @@ def load_slp(filename: str) -> Labels:
     return slp.read_labels(filename)
 
 
+def save_slp(labels: Labels, filename: str):
+    """Save a SLEAP dataset to a `.slp` file.
+
+    Args:
+        labels: A SLEAP `Labels` object (see `load_slp`).
+        filename: Path to save labels to ending with `.slp`.
+    """
+    return slp.write_labels(filename, labels)
+
+
 def load_nwb(filename: str) -> Labels:
     """Load an NWB dataset as a SLEAP `Labels` object.