Apply the structural changes to master branch, namely:
The use of CoreState class as a container for core state quantities (implements #19 (closed))
Depreciation of ComputationContext (closes #9 (closed)) in favour of ComputationConfig that only stores relevant parameters of the config files (no matrices)
The use of Forecasts and Analyses classes that inherit from ComputationStep.
Easy choice between Python and Fortran implementation for each step
Python implementation is now the default one and wrappers were implemented (build_legacy...) to adapt to Fortran syntax rather than the other way around.