This function creates a poset for use in a pim model based on a
number of observations and a comparison type. This function is
new.pim.poset and returns a list that
can be used as a value for its argument
a character value, either 'unique' or 'all'
an single integer value indicating how many observations there are in the model.
A named list with 2 elements, called "L" and "R", containing the selection indices for the left hand and right hand side of a pim.