[Branch]: differential_rotation

This is the feature branch that contains some diff rot code, I think it needs some love so I am opening this so people don't forget about it!!
No, you just need to do:

```
assert_quantity(input1, input2, input3)
```

the function (which should live in util) could be something like this:

```python
def assert_quantity(*args):
    for arg in args:
        if not isinstance(arg, Quantity):
            raise TypeError("Input should be an Astropy Quantity object")