Added zGetExtra, zSetExtra, lenScale, calculateHiatus

Added API functions zGetExtra(), zSetExtra. Also added convenience
functions lensScale() and calculateHiatus(). The lensScale() convenience
function is not complete yet. the calculateHiatus() function was
basically moved from Examples\searchLargestHiatusLens.py to pyZDDE.py
Added minor changes to searchLargestHiatusLens

Added the ability to "scale lens design" before performing the search.
Also moved and converted the code block for calculating Hiatus from
searchLargestHiatusLens.py to a convenience function of pyzdde class.