Add quo_rem method in class Polynomial_generic_sparse
Move inversion of unit
Add doc tests
Trac #16544: Implement `quo_rem` for `Polynomial_sparse_generic`

It is not possible to compute the Euclidean division for generic sparse
polynomials.
{{{#!python
sage: R.<x>=PolynomialRing(ZZ,sparse=True)
sage: p=R.random_element(10)
sage: q=R.random_element(5)
sage: p//q
------------------------------------------------------------------------
---
AttributeError
...
AttributeError: 'Polynomial_generic_sparse' object has no attribute
'quo_rem'
}}}

URL: http://trac.sagemath.org/16544
Reported by: bruno
Ticket author(s): Bruno Grenet
Reviewer(s): Travis Scrimshaw