Mark stuff that shouldn't be reused

[janbrohl]

By @dhellmann :
We should also ensure that we are only exporting symbols we want someone else to import at the API level in the python code. Renaming some modules to prefix them with _ would signal they shouldn't be used directly.

[dhellmann]

#58 is a prerequisite for this work.

[janbrohl]

Why?

[dhellmann]

I figured we would want good tests in place before we start rearranging code and renaming things.