Move `compute_assimilation` into FF16 Strategy as a lambda

[aornugent]

Draft of changes to modularise how assimilation is calculated. Where previously the per-leaf photosynthetic canopy was hard-coded, it now resides in FF16_Strategy::compute_assimilation which is passed to assimilation::assimilate for integration along the distribution of canopy openness during calculation of net_mass_production_dt.

I've left the leaf level approximation in assimilation for now, which can be accessed from the assimilator object -

// ff16_strategy.cpp
double FF16_Strategy::compute_assimilation(double z, double height,
                                     const FF16_Environment& environment) {
  return assimilator.assimilation_leaf(environment.get_environment_at_height(z)) * assimilator.q(z, height);
}

When FF16w comes online, we can substitute in a new method while also reusing q to access information about the distribution of leaf area -

// ff16w_strategy.cpp
double FF16w_Strategy::compute_assimilation(double z, double height,
                                     const FF16_Environment& environment) {
  // something like..
  return leaf::assimilation(environment.get_environment_at_height(z)) * assimilator.q(z, height);
}

The only requirements are to return a double and scale the result by area_leaf:

// assimilation.h
double assimilate(std::function<double(double)> f, ...) {
    ...
    return area_leaf * A;
}

We can modify this last line if additional scaling terms are needed and set them to 1 in FF16 to maintain backwards compatibility.

[codecov-commenter]

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[aornugent]

Merged and extended in simple_water_model branch.