Improvements in WHO 0-24m and CDC 2-20y, and bugfix for GA calculator

Implemented new features:

• WHO 0 - 24 month growth calculator, added predicted increases in weight / head circumference / length to maintain current percentile, for both chronologic and corrected age
• CDC 2 - 20 year growth calculator, added ability to also display calculations for corrected age, for children who were born premature
• Gestational age calculator, updated to properly display age when > 1 year old

Thanks to the PediTools users who pointed out the problems and made suggestions for improvement!