Convert mBuild boxes to numpy arrays 
With https://github.com/mosdef-hub/mbuild/pull/672, some tests in [test_convert_mbuild.py](https://github.com/mattwthompson/topology/blob/master/topology/tests/test_convert_mbuild.py) are not passing and give the error message: 
```python
unyt.exceptions.InvalidUnitOperation: Tried to multiply a Unit object with '[0.22807001 0.20428    0.18468   ]' (type <class 'mbuild.box._BoxArray'>). This behavior is undefined.
```
I think this because mbuild.Box attributes are type `BoxArray` instead of `ndarray` and `unyt` doesn't recognize `BoxArray`s. 


Probably the easiest fix would be to cast the BoxArray as an numpy ndarray before we apply units. In this line for example:
https://github.com/mattwthompson/topology/blob/91e527bf5b9da516cb0788247070f58757927f5f/topology/external/convert_mbuild.py#L87

diff --git a/topology/external/convert_mbuild.py b/topology/external/convert_mbuild.py
index 6437836..e30e8ed 100644
--- a/topology/external/convert_mbuild.py
+++ b/topology/external/convert_mbuild.py
@@ -142,8 +142,8 @@ def from_mbuild_box(mb_box):
         return None
 
     box = Box(
-        lengths=mb_box.lengths*u.nm,
-        angles=mb_box.angles*u.degree,
+        lengths=np.asarray(mb_box.lengths)*u.nm,
+        angles=np.asarray(mb_box.angles)*u.degree,
     )
 
     return box
['topology/external/convert_mbuild.py']