Changed SPDE from recursive to iterative

changed SPDE function from recursive to iterative as it is called in most of the other functions like(rischDE ,paramrischDE etc)  which on a recursive call makes it difficult to compute ouput because of recursion limit getting exceeded
replaced lines like pow_a = pow_a + 1 with pow_a += 1
Adds test case to test_rde(test_spde)

adds a test case to check *if not c.rem(g).is_zero:  # g does not divide c* raising a NonElementaryIntegralException
Merge pull request #1978 from cheatiiit/try

Changed SPDE from recursive to iterative