Convert Linear and Quadratic to use parent PolynomialWeightedRandom

This makes some small but significant changes to the API and behaviour:
 - LinearWeightedRandom now requires slope before base in constructor,
   so that polynomial coefficients are consistently specified from
   greatest degree to least degree.
 - Polynomials are calculated from zero, so if a coefficient is not
   provided for the term of zero degree, the first value will have no
   weight.
   This has little effect on linear functions (if the base is left at
   the default value of one), but quadratic weights will differ since
   they were previously calculated as f(x) = (x+1)^2
   (previous default: [1,4,9,16] current default: [1,5,10,17])

Also set the number of iterations to use in unit tests via a constant
in the PHPUnit config file.