Enable option to select between mu scaled variance and static variance

[gowerc]

The current longitudinal model implementation is:

$$ y \sim N( \mu_i,\ \mu_i\ \sigma) $$

A common alternative is:

$$ y \sim N( \mu_i,\ \sigma) $$

We should have an option to support this.

Needs to be implemented for:

• [x] GSF
• [x] SF
• [x] Claret Bruno