use consistent Problem/Solution/Conclusion chapter format

[ngauthier]

Each chapter should have the following three sections:

. The Problem . The Solution . Conclusion

Always use a subsection under each of these sections for proper 3-digit section numbering.

[ngauthier]

Status: almost all the chapters have been revised to fit:

• Introduction: One sentence to one paragraph (max) describing what the pattern does
• The Problem: a section describing when the pattern is appropriate to apply, optionally with naive implementations
• The Solution: a section describing the application of the pattern

Only a few chapters left that have this problem and they are on @eee-c's plate.