Make `orientnew` respect parent class

With current SymPy stable:

```
In [1]: from sympy.physics.mechanics import ReferenceFrame

In [2]: class MyReferenceFrame(ReferenceFrame):
   ...:     pass
   ...: 

In [3]: A = MyReferenceFrame('A')

In [4]: B = A.orientnew('B', 'Axis', [0, A.x])

In [5]: type(B)
Out[5]: sympy.physics.vector.frame.ReferenceFrame

```

I expected `B` to be of the same class of `A`. I use this a lot because I usually overwrite the default versors to something like

```
class IJKReferenceFrame(ReferenceFrame):
    def __init__(self, name):
        super().__init__(name, latexs=['\mathbf{%s}_{%s}' % (idx, name) for idx in ("i", "j", "k")])
```

The needed change is minimal and is contained in this pull request. Probably I should write some kind of tests, I can do them if needed when I have 20 spare minutes by next week.