splat.gen: fix Generator.levels.setter checks
splat.interpol.Polynomial: define coefs, derivative and integral

* rename .pol -> .coefs attribute as it's really the
  only the coefficients
* make .derivative a method to always return a new Polynomial
  instance (and avoid bad surprises)
* add .integral with optional constant to also return a new
  Polynomial but with integral coefficients
test: add test_polynomial for Polynomial class
rain.py: update to latest Polynomial interface
add splat.plot with function to plot a spline
splat.interpol.Spline: convert all points to float