Add Mod._eval_is_nonnegative/nonpositive helpers

closes #8581
to_rational_coeffs: return None if all numerical coeffs are Rationals

    Do not return something if numeric coefficients are already
    Rationals or else a recursion error at Line 974 of polyroots will
    ensue.

    Formerly, the presence of a sqrt(non-number) was giving a return
    value which lead to the recursion error under certain conditions.
Fix Pow._eval_is_rational for integer exponents

closes #8598
UnXFAIL some diffgeom tests in the test_args.py
Abs evaluates as integer/rational when needed
Merge pull request #8590 from skirpichev/8581-fix

Add Mod._eval_is_nonnegative helper
Merge pull request #8599 from pelegm/abs_assumptions

Abs evaluates as integer/rational when needed
Merge pull request #8600 from skirpichev/8598-fix

Fix Pow._eval_is_rational for integer exponents
Point to versioned docs in the greeting message of isympy/live shell

fixes #8012
Change version to 0.7.6-git
Fixes simplification of more than two summations
Merge pull request #8608 from shivamvats/shivam/8596

Fixes simplification of more than two summations
Fix 1**z evaluation if z is infinite

closes #8582
Fix evaluation of Abs(oo + imag) or Abs(real + I*oo)
Added the definition of sech

Definition of csch will follow in further commits.
Added the definition of csch

Also made some corrections and changes in sech and added the inverse function
of cosh.
Created a reciprocal class and added tests

1) Put sech and csch in a ReciprocalHyperbolicFunction class
to avoid code duplication
2)Added tests in core/test_args and for some basic functions
Modified tests after resolving merge conflicts
Moved some more functions to the Reciprocal class

Moved rewrite_as_tanh and rewrite_as_coth to RecicprocalHyperbolic
class.
Fixed broken TeX and tests and merged Mathematica's translations

Also did away with redundant tests