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.
keep floor(i) unchanged if i is imaginary symbol
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