Coordinates ape5 rep tests

@astrofrog @Cadair - this fixes up the APE5 api test to work with the representations code.  It's mostly relatively minor things, but there's one issue as yet un-resolved: why doesn't:
```
cr = CartesianRepresentation(x=randn(100)*u.kpc,y=randn(100)*u.kpc,z=randn(100)*u.kpc)
cr.x.kpc
```
work?  Shouldn't ``cr.x`` be a `Distance` if it has length units?