Indent an XML document by inserting newlines and indentation space
after elements.
*tree* is the ElementTree or Element to modify. The (root) element
itself will not be changed, but the tail text of all elements in its
subtree will be adapted.
*space* is the whitespace to insert for each indentation level, two
space characters by default.
*level* is the initial indentation level. Setting this to a higher
value than 0 can be used for indenting subtrees that are more deeply
nested inside of a document.