rename 'unlabeled_label' -> 'unlabeled_class'; define as constant and use constant for default args

[NickleDave]

because it's confusing to talk about "labeled as unlabeled", and by analogy with the "background" class used in object detection

• define a constant in vak.constants like BACKGROUND_CLASS_NAME = 'background'
• use that constant in all the functions that have it as a default, e.g. vak.labels.multi_char_labels has a skip default that currently is ('unlabeled',), rewrite as -> (constants.BACKGROUND_CLASS_NAME,)
• also use in vak.labelset itself when we make the labelmap

[NickleDave]

Thinking about this more.
The thing is that "unlabeled" is the more precise name.
When we use it, it's because we specifically care about segments left unlabeled when annotation a sequence of segments.

But I agree with myself that

• unlabeled_label is a confusing variable name, should be renamed unlabeled_class
• it would be better to have a constant and use that throughout the code base instead of having to find

[NickleDave]

Done in #755. Note I went with background_label