The module lacks a 'where' procedure.
I was looking for a procedure similar to `numpy.where()` from python numpy but I cannot find it. So, I think it is beneficial to create a procedure that do this task, i.e., return the indices where the condition is true. 

https://github.com/numpy/numpy/blob/1b8b46b3f2f68f5be8e52e798eb91c2ac5952745/numpy/ma/core.py#L7249


