Changes presentation of Maturity Models.

These changes are in response to Von Welch's comment in the Google Doc tha a
"maturity model for scientific software would be generally useful, with
security as one example."
Includes rOpenSci example.
Removing subsections from software section

Looking at integrating several subsections as themes, but the
intent for these sections was to be integrated into the text.
Adding back in some of the subsections to software

Some reordering of text and discussion, fitting in the main text
with the overall themes of the discussion in order to make it easier
to read through the section.
Added some introduction to backward compatibility
Various edits in Policy.

Includes adding one aspect from discussion documented in Google Doc. The one
other aspect that is missing is "Is sustainability designed for or an emergent
property". However, I think this best goes into the "What does sustainability
mean" cross-cutting issue (if it isn't there already).