[ENH] experimental - mapping tags on attributes

[fkiraly]

This PR introdudes an experimental design where tags are mapped onto unoccupied attributes.

E.g., the value of the tag "foo", in an instance of a BaseObject child inst, can be queried by inst.foo instead of inst.get_tag("foo").

For tags that have special symbols, these are coerced to valid python variable names, replacing : by __, and - by _.