Remove description of strides from novice Python 01

The current section on slicing is already relatively long. Since
we don't use strides anywhere else in the novice Python material
this removes them to avoid overwhelming the students with too much
detail too early.
Updating markdown version of 01-numpy
Remove reference to strides in natural numbers exercise

The ability to skip over count in numbers other than one isn't necessary
for the exercise, so this presents the minimal amount of information
that the student needs.

This also allows us to remove discussion of strides from the discussion
of indexing in the 01 notebook letting us minimize the amount of extra
material we are delivering.
Merge pull request #692 from ethanwhite/remove-strides-again

Remove description of strides from novice Python 01