Read in some x-y data and then find a least-squares fit to the data as a linear combination of given functions, or as a nonlinear function.

Read in arrays `x` and `y` from xyL.dat and find the best 5th degree polynomial fit

     read xyL.dat x y
     scalar\fit a0 a1 a2 a3 a4 a5
     fit y = a0+a1*x+a1*x^2+a2*x^3+a3*x^4+a4*x^5

Read in arrays `x` and `y` from xyN.dat and find the best fit to `y(x) = a0*exp(a1*x)*sin(a2*x+a3)`

     read xyN.dat x y
     scalar\fit a0 a1 a2 a3
     a0 = 5
     a1 = -.5
     a2 = 1.5
     a3 = .5
     fit y = a0*exp(a1*x)*sin(a2*x+a3)

Read data from files
Define and plot a function