The dynamic is now located into vle.discrete-time

 [@Andrianasolo2014]

All parameters have to be given in vpz.

- rh1 and rh2 are initialisations of N1 and N3
- we use rather init_N1 and init_N3 params of discrete-time

boost thread and system were missing into requirements

CMakeCache.txt should not be versionned, it is generated by cmake and
differs form a computer to another.

removed the climatic files from model package. a symbolic link is used to call external files from package directory

water balance : modified default value for ETRETM (0). modified all vpz to new nitrogen dilution curve

modified nitrogen dilution curve parameterization according to [Debaeke2012a] for sunflo.vpz model (only)