Scaling of peak primary parasitemia

[JoRussell-IDM]

PPP_scale_factor should be calculated by a function with three default behaviors:

1. Either it reads a value from the config (PPP_scale_factor = 1.0)
2. If InfectionMalaria2Config::use_fixed_scale_factor: then use the value for fixed_scale_factor in the config
3. Enagage a function that calculates a PPP_scale_factor based on the set of indiviudal parameters we deem important for

The first_peak_value in InfectionMalaria2::get_infection() is effectively scaled by PPP_scale_factor and that value still falls between the clamped min and max of InfectionMalaria2::get_infection()

[DGoes-IDM]

Here's where we're either calculating the scale factor or setting it from the config based on a flag:

If we're calculating it, here's how it's calculated:

[JoRussell-IDM]

Related to conclusion of #14