Rework of PR #1099 which is in turn a rework of PR #891 (by @RNKuhns), based on discussion recorded in #981 on tags.

Adds a BaseObject class which adds dynamic tag functionality. BaseObject contains BaseEstimator functionality sans fit state change related interfaces.
The new functionality replaces the inconsistent mixture of old interface points (all_tags, has_tag) in all downstream dependencies.

post-PR, tags work like this:

• each class will have "class tags", stored in the content of the class variable _tags: dict. Individual class tags are considered to override each other as per normal nested inheritance. E.g., if a parent class and the class have different values for a class tag, the younger class' class tag value counts.
• each object can, in addition, have dynamic tags, in the object variable _tags_dynamic: dict. These are expected to be initialized in the constructor, for instance in cases where behaviour described by tags depends on components or hyper-parameters. Dynamic tags are always considered to override the class tags - e.g., if the object's class or a parent class have a static tag of the same name, the dynamic tag's value overrides any class tag's value.

The new tag related methods make use of this inheritance logic are as follows.

class methods:

• get_class_tags() collects all class tags and their values and returns them as a dict (keys = tag names, values = tag values). It overrides values precisely as described above, in the return dictionary. This replaces the old _all_tags.
• get_class_tag(tag_name, tag_value_default=None) retrieves the value of an individual tag with name tag_name, and optionally allows to specify a default value if the tag is not found. The inheritance/override logic is the same as in get_class_tags.

object methods:

• get_tags() collects all class tags and dynamic tags and their values and returns them as a dict (keys = tag names, values = tag values). It overrides values precisely as described above, in the return dictionary.
• get_tag(tag_name, tag_value_default=None) retrieves the value of an individual tag with name tag_name, and optionally allows to specify a default value if the tag is not found. The inheritance/override logic is the same as in get_tags.
• set_tags(**tag_dict) sets dynamic tags according to keys (=tag names) and values (= tag values) in the argument dictionary. It is expected to be used in constructors typically.
• clone_tags(estimator, tag_set=None) is a shorthand version of set_tags, where it sets tags according to their get_tag values in estimator. The tags that are mirrored/copied in this way can be restricted by specifying a list of tag names in tag_set, otherwise all tags are copied over. The mirroring is by-reference and not by-value (if the type allows this), but tags are not expected to be mutated.