improved version of `berkovic_trees`

[swewers]

• a Berkovich tree now has an attribute _attr which is initialized as an empty dictionary. It is never use by this module, and may be used by applications to store arbitrary information related to a vertex
• fixed bug: T.children() now returns a copy of the internal list of children. To change the tree structure, one has to change T._children
• fixed bug: find_point now works for empty tree
• new function remove_point
• more examples
• fixed some typos

[coveralls]

Pull Request Test Coverage Report for Build 1589

• 22 of 24 (91.67%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.05%) to 76.599%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
mclf/berkovich/berkovich_trees.py	22	24	91.67%
<!--	Total:	22	24	91.67%	-->

Totals
Change from base Build 1579:	0.05%
Covered Lines:	3522
Relevant Lines:	4464

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💛 - Coveralls