Protocol: Remove slot requirement for containers

[Yuffster]

Protocols should be able to accept containers with either a label or a slot or both.

p = Protocol()
p.add_instrument('A', 'p10')
p.add_container('microplate.96', label="Ingredients")
p.add_container('microplate.96', label="Output")
p.transfer('Ingredients:A1', 'Output:A1')  # OK

p.transfer('A1:A1', 'B1:B1')  # Not OK; slots are unknown.

This will allow more dynamic Protocol concatenation, such as...

p1 = Protocol()
p1.add_instrument('A', 'p10')
p1.add_container('microplate.96', label="Ingredients")
p1.add_container('microplate.96', label="Output")
p1.transfer('Ingredients:A1', 'Output:A1')  # OK

p2 = Protocol()
p2.add_instrument('A', 'p10')
p2.add_container('microplate.96', label="Samples")
p2.add_container('microplate.96', label="Output")
p2.transfer('Samples:A1', 'Output:A1', ul=10)

p3 = Protocol()
p3.add_instrument('A', 'p10')
p3.add_container('microplate.96', label="Ingredients")
p3.add_container('microplate.96', label="Samples")
p3.add_container('microplate.96', label="Output")
p3.transfer('Ingredients:A1', 'Output:A1')
p3.transfer('Samples:A1', 'Output:A1', ul=10)

(p3 == p1 + p2)  # True

[Yuffster]

This should also apply to exported JSON (requiring a change to the JSON format) and documented as its own issue #9.

{
    "containers": [
        {
            "name": "microplate.96",
            "label": "Ingredients",
            "slot": "A1"
        },
        {
            "name": "microplate.96",
            "label": "Output"
        }
    ]
}