Adding basic MetricSignature functionality

[cleong110]

Breaking this out of #4. Following the conventions of sacrebleu, implementing Signature class.

[AmitMY]

i suggest extending it via

self.signature_info = {"name": name, **args}

that way, it is easily extendable. also, higher_is_better never has None

[cleong110]

It can have None if it is not a Metric signature, but a Signature of, say, a DistanceMeasure, or an Aggregator or so forth

[cleong110]

Or that is to say, some Signatures might not have the "higher is better" field at all, once we extend the use past Metrics

[cleong110]

The {**args} idea does seem good! Tried implementing it that way, and realized that if the subclass defines something like kind, but doesn't define an abbreviation, we get a crash. Hmmm

[cleong110]

Pushed a new version. This is what the full and short signature looks like when you do BaseMetric("base") and DistanceMetric()

Note how DistanceMetric defines kind but doesn't have a DistanceSignature yet to define how to abbreviate it, etc.

**********
name:base|higher_is_better:no
n:base|hb:no
**********
name:DistanceMetric l2|higher_is_better:no|kind:l2
n:DistanceMetric l2|hb:no|kind:l2