QQbar cleaning 2

[vdelecroix]

We further simplify QQbar code by:

• using python operator to indentify binary operators instead of strings. In other words we replace '+' by operator.add, '-' by operator.sub, etc
• writing only one function binop instead of addsub and muldiv.
• removing the methods kind, is_exact, is_rational, is_field_element of descriptors and instead use the class themselves
• removing the method rational_value and directly access the _value attribute
• detect unions earlier in the code to avoid constructing ANBinaryExpr

As a consequence of the last items, we have exactification detected earlier

sage: sqrt17 = QQbar(17).sqrt()
sage: sqrt19 = QQbar(19).sqrt()
sage: (sqrt17 + sqrt19).exactify()
sage: sqrt17 * sqrt19 + sqrt17 - sqrt19 * sqrt17 - sqrt17
0

Instead of 0.?e-17 on the current beta. See also the better output in doctests from the commit 012ae18.

follow up: #19955

[vdelecroix]

Last 10 new commits:

​b35d6f6	Trac 19954: fix doctests
​9f39b58	Trac 19954: fix sage_input doctests
​7ab3ffb	Trac 19954: move the 34-gon as a doctest
​ec2e9aa	Trac 19954: merge 7.1.beta1
​d1788a0	Trac 19954: fix doctests
​94629db	Trac 19954: doctest independent of execution order
​cb2592c	Trac 19954: typo in documentation
​1385753	Trac 19954: documentation
​c1f5c5f	Trac 19954: remove useless "gaussian" functions
​9264ff1	Trac 20074: qqbar cleaning

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​012ae18

Trac 20074: fix doctests

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​ff5eccc	Trac 20074: merge 7.1.beta3
​c23b72b	Trac 20074: fix doctest
​66b22b5	Trac 20074: simplify descriptors