Working Kerr effect; PDM; speedups; removed unused files

Working Kerr effect; PDM; speedups; removed unused files

Kerr effect is now working, showing the expected curve shape as Tx
power varies. Currently this uses a fixed step size of 1km, with
attenuation, and amplification every 50km. Amplification will be removed
later to simulate a true passive network, but now kept to verify results.

Since non-linearity is now introduced, we change the Tx signal to
having 8 samples/sec (previously 2). The Rx then downsamples at the
Rx filter.

"channel.m" is now added to simulate the overall channel with all
effects considered. Currently this includes Kerr, CD and phase
noise. PDM was then implemented, with "pdmchannel.m" being the
PDM analogue of "channel.m", with corresponding new files for the
PDM version of the CMA equalizer and the split-step solver.

Simulations are now significantly faster by doing all filtering
in the frequency domain (so multiplication instead of convolution).
The behaviour is changed slightly: instead of zero initial conditions,
we now take periodic (circular) boundary conditions. Simulation
lengths were also changed to an integer power of 2 to have the
most efficient FFT.

Some initial testing files that are no longer needed are removed.