Added a Python PDD modeling tool

PyPDD is a small Python PDD model that I wrote in the last days. For short documentation see:

https://github.com/jsegu/pypdd

I believe it does what PISM should do after issue #133 is fixed, except that it handles only monthly data, does not (yet) uses the Calov & Greve integrand, and integration is very simple (numpy.sum).
Added a PDD model test (details bel w)

The test runs:

    pypdd.py -b -o pypdd_smb.nc
    pisms -Lx 1 -Ly 1 -Mx 21 -My 21 -y 0 -o i.nc
    pclimate -i i.nc -o pism_smb.nc -times 0:1:1 -atmosphere given
      -atmosphere_given_file atm.nc -atmosphere_given_period 1
      -surface pdd -pdd_annualize -pdd_std_dev 0.01
    nccmp.py -v climatic_mass_balance pypdd_smb.nc pism_smb.nc

This produces four files:

* atm.nc, containing a fake climate generated by pypdd.py;
* i.nc, containing input for pclimate;
* pypdd_smb.nc, with the resulting mass-balance from pypdd.py;
* pism_smb.nc, with output from pclimate.

The fake climate in atm.nc is defined by:

    temp(t, x, y) = -10 * (y + cos(2*pi*t))
    prec(t, x, y) = x * (sign(x) - cos(2*pi*t))

With x, y in km and t in yr. You can think of it as a squared geographical domain where:

* temperature amplitude is the same (10°C) everywhere
* temperature always peaks in summer
* mean temperature increases from North (-10°C) to South (10°C)
* min precipitation is zero everywhere
* mean precipitation is zero in the center line of the domain, and increases to 1 m/yr in the western and eastern edges.
* precipitation peaks in winter in the western (maritime) domain half and in summer in the eastern (continental) domain half.

Expectedly, smb.nc and pism_smb.nc differ. Notably, pism_smb.nc is y-symetric, which reflects PISM's insensitivity to precipitation seasonality. This is most obvious in the accumulation field and consistent with issue #133.