Allow adding objects with a 'protocol' attribute to protocols.

[kalekundert]

This could be a convenient idiom:

@autoprop
class Pcr:
    def get_protocol(self):
        p = stepwise.Protocol()
        ...
        return p

p = stepwise.Protocol()
p += Pcr()
p.print()

[kalekundert]

Fixed by 953536d.