November 2014 changes on maintenance respiration and others

[gas-exchange.cpp] Set min RH to 25%
[plant.cpp] Removed age effect on maintenance respiration

These changes are additional to the October changes that haven't been
merged:

[leaf.cpp] Senescence rate in response to water stress is relaxed by a
half
[Development.cpp] Temperature during induction period is separated as
T_ind.