CREATE - create model, boundary edge triangles, edge face, edge vertex, add
         a vertex

makeModel		Make and return a pointer to a new model
makeBoundaryEdgeTri	Create a boundary edge and triangle 
makeEdgeFace		Create an edge by connecting two vertices
makeEdgeVertex		Create an edge connecting an existing vertex (v1) to a
                        new vertex
addVertexToModel	Add a vertex to model, and return pointer to new vertex
insideTriInModel	return pointer to triangle in model containing
                   	specified (x,y) coordinates

Function Prototypes:
Model *makeModel (float xmin, float ymin, float xmax, float ymax);
void makeBoundaryEdgeTri (Vertex *v, Edge **enew, Tri **tnew);
void makeEdgeFace (Vertex *v1, Vertex *v2, Edge **enew, Face **fnew);
Vertex* addVertexToModel (Model *m, float x, float y);
Tri* insideTriInModel (Model *m, Tri *start, float x, float y);

makeModel:
Input:
xmin		minimum x-coordinate
ymin		minimum y-coordinate
xmax		maximum x-coordinate
ymax		maximum y-coordinate

Returns: pointer to a new Model

makeBoundaryEdgeTri:
Input:
v		specified boundary Vertex

Output:
enew		new boundary Edge
tnew		new boundary triangle

Notes:
The specified vertex and the adjacent vertices on the boundary
are assumed to be colinear.  Therefore, the resulting
boundary triangle has zero area, and is intended to enable
deletion of the specified vertex from the boundary.

makeEdgeFace:
Input:
v1		First Vertex
v2		second Vertex

Output:
enew		new Edge
fnew		new Face

Notes:
The vertices must be adjacent to a single common face.
This face is closed off by the new edge, and a new edge and
a new face are made and returned. 

addVertexToModel:
Input:
m		model
x		x-coordinate of new vertex
y		y-coordinate of new vertex

Notes:
If the new vertex is close to an existing vertex, this function returns NULL.

insideTriInModel:
Input:
m		Model
start		triangle to look at first (NULL to begin looking anywhere)
x		x-coordinate
y		y-coordinate

Notes:
Points on an edge of a triangle are assumed to be inside that triangle.
An edge may be used by two triangles, so two triangles may "contain"
a point that lies on an edge.  The first triangle found to contain
the specified point is returned.

Author:  Dave Hale, Colorado School of Mines, Fall 1990.