Add `key` field to AxisArray structure, with a Unit to set the key and another to filter on key

[cboulay]

(ignore array_chunker commits)

Closes #142

This PR adds a .key field to AxisArray then adds a couple functionalities related to it:

• AxisArray.concat(..., filter_key="some key") will only concatenate messages with matching keys
• AxisArray.concat(..., filter_key=None) (default) will not filter but will set key to "" (default) if keys are not matching.
• set_key generator or SetKey Unit sets the .key field to a new value.
• FilterOnKey (Unit only) only allows through messages with matching .key value.

[cboulay]

Close in favour of #146