Accumulate addon

[tfarago]

Stores experiment data so one can easily and efficiently access all of it as numpy arrays.

Todo:

• make the interface more compact
• make sure memory is released on detach

Requires #376. With this addon we can do:

ex = Experiment(...)
acc = Accumulator(ex.acquisitions)
ex.run().join()
backproject.insert(acc.items[ex.radios][:, height, :])

[tfarago]

@matze this one is done too.

[matze]

👍