Make attributes a guarded property

[pp-mo]

As described in this comment
In place of a regular "NameMap", we have a controlled map with a customised __getitem__ and __setitem__, ensuring the array-type content.

There is little doubt that overall this would be a smoother overall solution.
For now the problems are just :

• delays a v0.2 release a bit further
• (not least because..) requires more changes to documentation
• probably interacts with NameMap should work as a generic type #85 -- the other very desirable delayed change