Added allSAT
Added Model class.
Fixed variable naming.
Added returning of False for UNSAT theory.
Improved pl_true
added function for LaTex printing of Xor statements
Added documentation and tests.
give MaxMinBase an evalf routine

Although Or, And, etc... will not have a number as an
argument, Min and Max can. When the operations version
of _eval_evalf is called a call to as_independent is
made. This creates an x (=1) that is assumed to be an
argument of the object so Max(0,y).n() becomes Max(1, 0, y) -> Max(1, y).
To avoid this an evalf routine was added to handle MaxMinBase separately.
simplify test [skip ci]
Merge pull request #7928 from smichr/7233

give MaxMinBase an evalf routine
Merge pull request #7894 from SDB1323/pl_true

Improved pl_true
test_numbers: fix minor typo in a test so both >= and <= are tested

The test did `<=` twice.  Use `>=` for one of them.
Merge pull request #7941 from cbm755/fix_test_numbers_typo

test_numbers: fix minor typo in a test so both >= and <= are tested
update fastcache to most recent version in travis tests
Added yielding False for UNSAT theories.
Merge pull request #7946 from pbrady/update_fastcache

update fastcache to most recent version in travis tests
Printing: print x**-1.0 differently from 1/x

`x**-1.0` and `x**-1` should print differently.  Fixed this and added
tests.  Pretty printing seemed to work already but tests added.
Close #2860 which is related (it seems fixed but added tests).
Fixed sphinx error.
Merge pull request #7874 from SDB1323/AllSAT

AllSAT
6653: modify slow test but keep coverage

Before the original changes were made at 0fa7b9730e56759e5f4bc3752143f5,
the expression added in this PR would also fail in the same way, raising
an assertion. And if the assertion were removed, this wrong expression
was obtained:

1 + O(x**2)

After the changes, the expression covers all added code and gives the
following result:

1 - x**2/2 + O(x**3)