helpers for datadict

[wpfff]

• avoid deepcopy
• meta-object for easier data access

[wpfff]

@marcosfrenkel can you have a look?
Also -- where should the new data access object be documented?

two main changes:

• deepcopying was in many places, and it's very redundant (we do it anyway in all plottr nodes). so i removed copying as default from pretty much anywhere in datadict, except where you'd really want it (like when returning the structure of the data)
• there's a new attribute to datadict: data of field 'x' can now be accessed as data.d_.x -- this gives direct access to the values of field 'x'.

[marcosfrenkel]

Everything other than comment looks good, and even the comment is nothing too important.

[marcosfrenkel]

@wpfff, the documentation for datadicts currently live here and that is the repository where the new feature should be documented. I can update all of the sub-modules there so that the API sections of stuff shows the current code.

[wpfff]

@marcosfrenkel i fixed the one thing you mentioned. ready to approve?

[marcosfrenkel]

Looks good!