first commit for ticket 8723 : multiplication in elliptic curves
trac #8723 : cleaned the doc
Trac #14823: Category of (complete) discrete valuation rings/fields
Reviewer changes: delete is_cdvr functions and friends, add abstract_method to precision_absolute and precision_relative, add some tests
Remove is_unit() from sage.structure.element.Ring and move it to the category of Rings

Before, the default implementation of is_unit() was not picked up by some
fields, e.g., some residue fields. Now this works correctly.
Some docstring improvements in modules_with_basis.py
change to clumsy but nonambiguous wording
improving the docstring for triangular module morphisms
also, pyflakes said these are useless
clarification on combination of keywords
More documentation fixes.
more doc fixes
typo fixed
more grammar fixed
very minor change, just to ping the branch
Merge branch 'trac8723' into new8723
Merge branch 'master' into triangdoc
implement gen.getattr() using closures
reviewer's patch, makes sure that the parents are correct even with caching
enable single-argument gcd/lcm