Add relation getters on base model structs

[parnic]

This prevents having to check the pointer before accessing a relation.

It was already safe to access a nil R's Get() method, but this removes some of the extra typing and nil checks as a convenience.

Previously:

var office *models.Office
if employee != nil {
    office = employee.R.GetOffice()
}

With this helper:

office := employee.GetOffice()

It is much more useful, however, when accessing nested relationships. Instead of

var c *models.RelC
if m != nil && m.R.GetA() != nil && m.R.GetA().R.GetB() != nil {
    c = m.R.GetA().R.GetB().R.GetC()
}

we can do

c := m.GetA().GetB().GetC()