Refactoring _data_setter & Co.

[brisvag]

We should get rid of the assumption that all datablocks want to use a data attribute, and that they should all use _data_setter for that. It's either resctrictive or useless for how groublocks or simply more complex datablocks work.

I think all datablock where it's obvious and intuitive should have easy access through __setitem__ like now, but without forcing an internal api for how to do that. As for more complex stuff: we should probably implement a "smart" setter/getter; maybe:

Particles[2]

should return something like

[position, orientation, property]

[brisvag]

As we discussed, we are going to keep data and _data_setters. I will think about implementing a "smart" setter for MultiBlocks!