Debugging sample function

The sample function returns too many zeros.  This needs to be fixed.
Fixed bugs in the plot sample routine

The sample routine is now stable.  One possible addition will be to sort the
return values.
Added the population module

This module is meant to contain functions useful for figuring out the
population properties of stars.  The module currently contains a function to
randomly draw a stellar mass from an IMF.

The README file was also updated.
Fixed return bug

The code previously didn't return anything

The function name was also changed to be all lowercase.
Added WD initial-final mass relation function