Change definition of primitive types from functions to classes

[jsreid13]

Currently within the class Schematic the primitive types (node, channel and port) are defined using methods with the same name which add them to the NetworkX DiGraph with several attributes, however several of these attributes overlap (especially for port and node). Moving forward it would be easier to maintain the code if each of these primitive types were separate classes instead and then they can inherit these attributes from each other (this is also more consistent with how the original Java project does it). So in this case Port would extend Node, simply adding the attributes of a fixed pressure and a type of fluid coming in or out.

The next step after this is to create a new Node type that is an electrical port that has voltage and current attributes which is needed for the next step of simulating electrophoretic chips.