$$RPR = \frac{m} {m_{max}} * 100$$
where \(m\) is the number of classes and \(m_{max}\) is the (theoretical)
maximum number of classes.
RPR is an 'Diversity metric'. The metric calculates the percentage of present classes
in the landscape in relation to a (theoretical) number of maximum classes. The user has to
specify the maximum number of classes. Note, that if classes_max is not provided,
the functions returns NA.
Behaviour
Approaches RPR > 0 when only one class type is present,
but the maximum number of classes is large. Equals RPR = 100 when m = m_max