integrals: An amendment to spde()

The iterative version of spde() currently assumes that the
coefficient  a  of  Dp  were invariant in the loop, in which
case the values of the multiplier  alpha  are powers of  a.
Otherwise the offset  beta  of the solution will be incorrect.

I have added two test cases where  a  is not invariant.
In the general case,  alpha  can be used instead of a power of  a
to construct the correct value of  beta.
product should return 1 if term=1, fixes #8301
doit() keeps unevaluated Product, if _eval_product returned nan
Add rational kwarg for Mul.as_coeff_mul, like for as_coeff_Mul
Improve evaluation of relationals with -oo if "other" arg is positive

fixes #8449
Merge pull request #8460 from skirpichev/8449-fix

Improve evaluation of relationals with -oo if "other" arg is nonnegative
Relational.canonical simplifies Ge -> Le and Gt -> Lt

This is made part of Relational simplification.

The presence of Relationals in Booleans sent to equals
raises an error since that routine is not set up to handle
them.
And and Or are complementary-arg aware

In addition, trivially equivalent forms like x > 1 and 1 < x are
recognized.
Implies is complementary-arg aware
Xor is complementary-arg aware
Equivalent is complementary-arg aware
8413: Min and Max are floor and ceiling aware

- facilitate changes with reversed property of Relational which
  reverses the direction of the Relational
special-case +/-oo in _Inequality
put Number on rhs when simplifying Relational

This will allow any test to be written as 0 < x or x > 0 since the former
will turn into the latter by Python rules (unless S(0) < x is written).
do not evaluate in reversed
rework connectivity testing; watch for evaluation
do not accept serendipitous evaluation in And/Or
add coverage tests
make Number on rhs part of Relational.canonical
do not use canonical unconditionally in simplify