Create poly.c

The following functions have been added:
struct poly *polyCreate()                                            //creates a polynomial
struct poly *polyDelete(struct poly *p)                              //deletes a polynomial
struct poly *polySetCoefficient(struct poly *p, int i, double value) //sets a given coefficient of a polynomial
double polyGetCoefficient(struct poly *p,int i)                      //returns a coefficient
int polyDegree(struct poly *p)                                       //returns the degree
struct poly *polyCopy(struct poly *p)                                //creates another polynomial
struct poly *polyAdd(struct poly *p0,struct poly *p1)                //adds two polynomials
struct poly *polyMultiply(struct poly *p0,struct poly *p1)           //multiplies two polynomials
double polyEval(struct poly *p, double x)                            //Evaluates a polynomial given a value x
void polyPrint(struct poly *p)                                       //prints the polynomial