Simplify Lifetime Management

[Steven-Roberts]

I'd like to propose the following changes to simplify the lifetime management of objects. In short I want to avoid all cases of an object not having control over the lifetime of objects it uses (except for VariableArrayView). Let me know if you have any concerns, objections, alternative ideas, etc.

• Remove reference member variables like
  

  SPAECIES/rainshaft/evaporation.hpp

  Line 406 in 3df7ba2

  const SaturationFormulae &sat_form;

  
  and instead store by value and accept in constructor by value. If the type is polymorphic like
  

  SPAECIES/rainshaft/limiting_integrator.hpp

  Line 26 in 3df7ba2

  const RainshaftIntegrator &integrator;

  
  I think using std::unique_ptr<...> would be idiomatic.

• For SumProcess use std::vector<std::unique_ptr<RainshaftProcess>> rather than std::vector<const RainshaftProcess *> (Related to Remove raw pointers #36)