Move section to a separate python module

We anticipate that the number of sections will grow, so this is a
good time to put it in a separate module
Add section factory and use it in core.py
class Section: Add export function

This defines an export function that can be used by all subclasses
by defining all exportable files as a list called self.export_files.

Note: This is yet another patch that does not add multiple inheritance
VerilatorSection: add export() method

This add an export() method to VerilatorSection, with the purpose
of exporting a list of files to be used in core.py.
core.py is also updated to make use of this method.