Have a method from_abstract_repr in Device to deserialize an abstract Device

[a-corni]

At the moment, users wanting to deserialize the abstract representation of a Device have to do
from pulser.json.abstract_repr.deserializer import deserialize_device followed by deserialize_device(abstract_device). It would be simpler to have Device.from_abstract_repr(abstract_device).

Initially suggested by @nataliejpg, useful for users of pulser_myqlm to build the device associated with the current state of the QPU (provided as a String, in the abstract format).