Merge pull request #2 from guangmingyao/black_dev

Black dev
Merge pull request #4 from guangmingyao/yao_dev

Yao dev
Merge pull request #6 from guangmingyao/master

units in double tank problem is changed to gram, rate out for tank 2 is changed to 4 l/min
Merge pull request #10 from guangmingyao/yao_dev

Yao dev
Changed a section number for one lecture
Bring master branch current to 2013.
Changed the order of the labs.
Minor wording to a problem involving an integrating factor.
Changes in the wording for the introductory modeling problems.
Added a section on variation of parameters for the 2nd order ODEs
Added a first draft for a new activity on second order DEs
Added a second part to the new worksheet.
Added an image to demonstrate the connections within the course topics.
Rearranged the activity on modeling.
Updated the modeling activity to include a problem on energy in the system.
Minor formatting changes for the 2nd order modelign activity.
Merge pull request #39 from KellyBlack/master

This is to bring the mast branch current to the final 2012 notes. This will allow us to make changes focused on 2013 for the dev branches.
Merge pull request #40 from KellyBlack/black_dev

Merge additional activities and a reordering of the activities based on the new schedule.
Merge branch 'black_dev' of https://github.com/KellyBlack/ODE-Recitation-Activities
change the forcing term in linearModeling.tex, it was not possible for students to integrate