Added presentation; DE-QPSK; CD with FFT; split-step Fourier

Added presentation; DE-QPSK; CD with FFT; split-step Fourier

LaTeX code and images for the 1st presentation (presented last
week) were added to the repository. Probably useful when writing
the technical milestone report.

Differentially-encoded QPSK was implemented last week. The results
were satisfactory, with smaller penalty than DPSK, and not affected
by cycle slips.

Chromatic dispersion is now (also) implemented in the frequency
domain. This gives a complexity of O(n log n) (set by FFT),
better than in the time domain (convolution, O(n^2)). This was
done as a prerequisite to implementing the split-step Fourier
method for simulating non-linearities (Kerr effect).

The split-step Fourier method was implemented, although not
thoroughly tested and may contain mistakes. To be discussed
with supervisor.

As discussed in last week's meeting, the code (at least the
CD code) was updated to only use 2 samples per symbol (sps).

Some convenience functions were added to aid debugging. They
may be removed later.