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.
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.
Merge pull request #3 from dsrivallabha/master

Added new ODE assignment:Predator Prey Model