TST: Test for selem.cube

Add test case for selem.cube

Also begs a minor question -  for organizing test cases while tracking. Would it be better to write test cases/functions in same order as they are in module? 
Like if the order of functions in selem.py is 
```
selem.square()
selem.rectangle()
selem.disk()
```
should the test cases for each of these also be in same order? This is just a personal preference but can be ignored depending upon what others think..