Install acquisition as attribute

Makes acquisitions within an experiment accessible as attributes which makes the usage of the whole thing much easier. This goes hand-in-hand with the read-only restriction on acquisitions and allowing access only by the specified `add` and `remove` interface. Eyecandy:
```python

e = FancyExperiment(...)
# Let's say the experiment defines darks and flats acquisitions, then this is possible
e.swap(e.darks, e.flats)
# This as well
e.remove(e.flats)
# Forgot radios?
radios = Acquisition(...)
e.add(radios)
```