Two-phase linear regression model

calculates the parameters of unidimensional two-phase linear regression model

INPUTS:
x - vector row with 'x' values
y - vector row with 'y' values
r - expected 'x'-coordinate of break point
if r is empty it is calculated during
the optimisation
p - if p is equal to 1 the fit is plotted
OUTPUT:
th - estimated paremeters of the regression
lines
y_1 = th(1) + th(2) * x
y_2 = th(3) + th(4) * x
r - the estimated break point
SSR - Sum of Squares of the Residuals
SSM - Sum of Squares due to the Model
R^2 can be calculated as RRM/(SSM + SSR)
res - vector of residuals