Module Modeling.PyModel

Pythonic Description of an OO-RDBM Model

Rules:
- a model is an object of type Model
- a model may specify a list of entities, each of type Entity
- an entity may specify a list of properties, that are either of type Attribute
  or \of type Relation
- all references to attributes, relations, entities, models, is by their "name"
  i.e. not by their columnName or className, etc, as the case may be
- whenever deemed useful, defaults are used
- note that the default "type" for an attribute is int 
  (not string -- seems more logical to me) 
- delete rule values are in lowercase

New Changes to note:
parentEntity -> parent 
Entity only exposes a properties[] (that holds all attributes and relations)
default for Model.adaptorName is None
added doc attribute on all description classes

Previous changes:
defaults class dictionary variable, for all model description classes
deleteRule -> delete
multiplicityLowerBound and miltiplicity upperBound -> multiplicity
parentEntity -> isAlso
default for displayLabel is value of name (not of columnName)
Entity only has attributes[] (that may also be relations)