rational = False , Default in Solvers. FixIssue#8515

I do suspect the flag is still needed, however. #8573 had a problem because some Floats were not converted to Rationals and when the equations were solved, the solution was missed. If we make rational=False be the default, then we will have people asking, "why doesn't this system (with Floats) solve?" and we will have to remind them to convert Floats to Rationals. For example:

```python
>>> solve(x**.5+x**.25-2,rational=False)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "sympy\solvers\solvers.py", line 909, in solve
    solution = _solve(f[0], *symbols, **flags)
  File "sympy\solvers\solvers.py", line 1414, in _solve
    "\nNo algorithms are implemented to solve equation %s" % f)
NotImplementedError: multiple generators [x**0.5, x**0.25]
No algorithms are implemented to solve equation x**0.25 + x**0.5 - 2
>>> solve(x**.5+x**.25-2)
[1.00000000000000]
```