Update assert statements

[cdce8p]

Assert will always throw a NameError. Adding the string just means that Python asserts that the string is true.
Therefore I would like to change the string to a comment.

>>> assert np, "numpy needs to be installed for this feature"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'np' is not defined