Uttrs

Bad version

More velocity on internal cohesion and conversion of array_accessors.

Now `uttr.ArrayAccessor` is not an *attrs class*. This prevents some bad behavior with the *attrs* helper funcions

This version implements a more strict version of `array_accessor()`

If you try to access an attribute no defined by *uttrs*, an `AttributeErrror` is raised.

This version provides the basic functionalities to use astropy units with attrs based classes.

Bugfix, now a None as default is working