Added new ODE assignment:Predator Prey Model

This new assignment in ODEs demonstrates use of Python to solve Predator Prey Model, also known as Lotka Volterra Equations.
Update Predtor-Prey-Model.ipynb

Added "%matplotlib inline" to code.
Update Predtor-Prey-Model.ipynb
Updated Predator Prey Model

Updated model to remove time from state space array.
Added "%matplotlib inline" command.
Added new file
1. Defined parameters and other variables as
 floats explicitly.
2. Updated documentation for RK4 scheme.
3. Added sections to notebook.
Start guidelines for contributions
1. Added arrow pointers to phase plot.
2. Added name and license terms as suggested by Prof Barba.
Added module number to file name, for easy reference.
Add new style for numericalmooc notebooks

Work-in-progress, according to discussions
Style: background removed
Tweak the style

Style for h6 now used in copyright notice.
Tweak style, h2
Add sketch of forces on a glider
Add formulation of no-drag case, with sketch
Tweak style sheet
enable equation numbers in MathJAX using javascript hack in CSS
Add equation numbers, fig. captions & polish text

To add equation number must enclose in an equation or eqnarray
environment
Used color schemer for headings

http://www.colorschemer.com/online.html
add notebook style section