First commit: creation of the file interval_posets.py with so basic classes in it
Creations of the interval-posets objects
Implementation of all basic methods to deal with Tamari interval-posets.

Especially, linking interval-posets to Dyck words and binary trees.
Changing "Dyck path" into "Dyck word"
Add a global inncludes and fix docs and test.
Adding a method to get a Tamari interval from binarytrees and Dyck words
Fixing the doc
trac #15683 code put into pep8 format, minor typo corrections
Fixing the cardinality test (was broken for n==0)
Adding some refs and docs + change cardinality to interval_cardinality
Merge branch 'public/combinat/interval-posets-15683' of trac.sagemath.org:sage into develop
disambiguate conflicting references, improve some doc
Some doc and adding a ``poset`` method to get the actual poset of the interval-poset
adding examples for the poset method
Merge branch 'develop' into tamari
def increasing_* reviewed; doc improved
own error fixed, complement interval-poset implemented
more edits, hopefully correct (Viviane?)
Adding the methods is_initial_interval and is_final_interval and some
docs and tests to the method complement
Merge branch 'develop' into tamari_new