FBG spectrum interface

I took Matthew's code and added what the spectrum of the FBG produced by
that Intensity patter would look like for a grating written centred at
1.55 microns and a refractive index variation proportional to the
normalised intensity modulation time 0.0002. The length of the grating
can be modified.  The FBG reflection intensity is normalised to its
highest peak. The reflection intensity is calculated using Rouard's
approach.