Doc/Links Update

3.1

Update models.rst

PopIn 3.0

• Introduced survival and mortal cdf
• debugged modified Weibull cdf from Chechenin
• added Mason's model
• added Hertz plot for F-h curves
• debugged loading of Excel file with empty or invalid sheets
• reorganized buttons on the GUI
• updated doc

PopIn 2.3

update README