MADAG protobuf support

[willdumm]

This PR introduces a system for subclassing HistoryDag according to expected node label data, and a standard way to convert between these subclasses. In particular, we add the following new subclasses:

• CGHistoryDag requires label data to include a compact_genome field, which contains instances of the new class CompactGenome. The new module containing this subclass, mutation_annotated_dag, includes interfaces to load and write to MAD protobuf, and to JSON. A CGHistoryDag can be created from a DAG that has node labels including a sequence attribute.
• SequenceHistoryDag requires label data to include a sequence field.

[willdumm]

The system for converting between HistoryDag subclasses is described in the HistoryDag and HistoryDag.from_history_dag docstrings. Provided sufficient label data is present, any HistoryDag subclass instance olddag can be converted to any new HistoryDag subclass type NewHDagType by doing NewHDagType.from_history_dag(olddag).

New HistoryDag subclasses need not override the static method from_history_dag, as long as they describe their required label fields and conversion functions in the static variable _required_label_fields. This variable contains a dictionary keyed by required label fields, each having as a value a list of tuples (from_labels, conversion_func), where conversion_func is a function taking a node and arbitrary keyword arguments, and returning the value of the required label field for that node, and from_labels is a tuple containing the names of label fields expected by conversion_func.

Conversion functions are required to accept keyword arguments, so that label data conversions which require additional data may be allowed. These keyword arguments may be provided to the from_history_dag method, and will be passed through to conversion_funcs.

One unresolved issue is that it is difficult to document the keyword arguments required for conversion to HistoryDag subclasses. It would be inelegant to rewrite the entire from_history_dag docstring even though the method itself needs no overriding.