Polynomial remainder sequences, gcd, and resultant

Documentation on the computation of gcd's and resultant's of
polynomials via remainder sequences, with emphasis on subresultants.
Computing resultant with subresultants

Reordered the loop in the computation of subresultants to include
one more scalar subresultant, namely, the resultant. Another loop
in the resultant function can then be omitted.

Notes:

1. The subresultants and the resultant of the pair `(f,g)` as computed
   are actually those of the pair `(g,f)` if `\deg(f) < \deg(g)`.
   This may have effect on the signs, but that is harmless in general
   when the resultant is only used for eliminating a variable.
   I have not changed this to save some time in the computation.
   Correct signs may be restored afterwards if that is necessary.

2. The resultant of `(f,0)` is zero indicating that `f` and `0` have
   common roots, namely every root of `f`. However, if `f` is a
   non-zero constant, it has no roots. In this case the resultant
   should be different from zero -- in fact one, by convention.
   I have not changed this as it would break the test in
   test_euclidtools.py.
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
Fix the flickering problem
Add rational kwarg for Mul.as_coeff_mul, like for as_coeff_Mul
Add test for issue 6134

fixes #6134
Add missing brackets while printing powers of Rational numbers

fixes #8409
Merge pull request #8418 from skirpichev/8409-fix

Add missing brackets while printing powers of Rational numbers
Temporarily comment out deploying docs (fixes #8387)
Merge pull request #8416 from skirpichev/6134-test

Add test for issue 6134
Merge pull request #8428 from skirpichev/8387-fix

Temporarily comment out deploying docs (fixes #8387)
Solved equality of arguments issue in integral.transform fix issue #8400
adjust whitespace
Add Sunny Aggarwal to AUTHORS/aboutus. Welcome to SymPy!
Merge pull request #8448 from sunny94/iss8400_transform

Solved equality of arguments issue in integral.transform fix issue #8400
Merge pull request #8376 from certik/issue3971

Fix the flickering problem
Fix evaluation of reciprocal trigonometric functions in even quadrants

fixes #8421
Merge pull request #8422 from skirpichev/8421-fix

Fix evaluation of reciprocal trigonometric functions in even quadrants