Add support for different endianness.
First pass at FITS support
Python 3 fixes
Add some comments about tests.
Make FITS schema more strict.
Various simplifications and refactorings
Add top-level schema
Remove extraneous print
Working state with nested schemas
Minor improvements
Add support for units
Simplify tree_walk
Fix infinite loop when no YAML end marker is present
Simplify basic tag writing
Fix how temporary tags work.
Add non-modifying walker
Remove extraneous prints