Add sub-sections to main User Documentation section
Move utils link into Nuts and bolts section
Move table/io.rst to top level and re-brand as Unified I/O
Add Table I/O as a top-level table feature
Merge pull request #967 from taldcroft/docs/index-sections

Add sub-sections to main User Documentation section and increase visibility of unified I/O interface
Change to common argument order in AngularSeparation
add test for AngularSeparation behavior
add exception module to utils
added warning for swaping lat/lon in AngularSeparation initializer
Added backwards incompatible changes section to What's New
Changed units-checking to work on real first argument