This directory is similar to @model, in that it only deals
with methods proper to the @model class. But we need a different 
directory from @model, to make those methods public, not private. 
The advantage of public methods is that they will use subsref and 
susasgn to access data in a @model object.

For ex: 
calling md.x in a "@model" routine just accesses the x field in the md 
structure. But calling md.x in a "model" routine will access the x field 
through the susref routine in @model. This ensures that we protect the 
data in @model classes from improper use.
