pretty-printing of aos #3

The default output is single-line, infix representation.

use Config.pprint_treelike=True to print in a tree-like format, with implicit and, or.

TODO: tree-like infix representation.

e.g. instead of
((gname & str) | (grp_id & str) | (rels & (rel_title & str)))
print

(gname & str) | 
(grp_id & str) | 
(rels & 
          (rel_title & str))