Checks for derivative in k(t)

Determines if a given function f in k(t) there exists u in k(t)
such that Du = f. Returns q such that i = f - Dq; v, c such that
i = Dv + cDt where v, c are in k; Output -> tuple (q, v, c) if there
exists a derivative for given function in k(t) else returns None
Removes Merge conflicts

But we cannot pass rational functions such as (t**2 - 1)/(t**2 + 1)
to such a function. Though according the definition the function
should work only for polynomial cases but the algorithm in
integrate_hypertangent would require us to solve even the test cases
of the form specified above
TestCase Addition: test cases for integrate_hypertangent
TestCase Addition: test cases added to is_deriv function
Solves following:

-> Adds XFAIL:
   Adds the test case which requires the CDS support in XFAIL
-> TFuncs in DE:
   Correct the Tfuncs in the DE in risch.py
Adds polynomial_reduce_kt

Polynomial reduce implemented in risch.py tries to find a, r
such that given a derivation D on k(t) and p in k[t]
where t is a nonlinear  q, r in k[t] p  = Dq  + r, and
deg(r) < deg_t(Dt)
Removes as_expr()

Tries to convert the code into rational function instead of passing
as polynomial. Also attempts to remove .as_expr() from the code
keeping the code bug free while passing through varoius methods
Addition of test cases(in risch.py)

This adds tests in risch.py to polynomial_reduce_kt to check if it
works in accordance with the previously implemented polynomial_reduce
(in risch.py)
Improvisations in integrate_hypertangent_reduce
and polynomial_reduce_kt
Test-Run : Run sympy/utilities