Revised leaf growth and senescence module

New leaf growth and senescence algorithms that substantially reduces the
use of GDD. A beta function based determinate sigmoid growth function is
used.  This function is driven by the simplified beta distribution
function using the same cardinal temperatures as in development class.

stayGreen parameter is also introduced before senescence begins. This is
assumed to be a fixed multiple of growthDuration since appearance of
each leaf. This parameter replaces longevity calculations using GDD.

Leaf area expansion and senescence appeared to work well. but overall
leaf DM decreases slightly which makes SLA to increase. Overall biomass
accumulation and allocation need to be reworked.

Many more changes have been made in my local version but committing the
minimal changes related to leaf module only for now. Future changes to
include gas-exchange calculation for each leaf.

But before making further changes, this version needs to be tested using
additional field data
Merge pull request #4 from cropbox-sk/master

Revised leaf growth and senescence module