Adding new band filters (see also Iwave.h):

 Note: the spectral range is from 250 nm to 4000 nm (see Iwave.h) which is 
   4000-250=3750nm. The step size is (4000-250)/1500=2.5 nm in the ffu.s array.

 o bands are defined by their discrete filter function
 o to compute a band position in the 1500 slots of ffu.s array, start
   with 250nm using 2.5nm increments. The result is the starting index
   for the spectral band.
 o 1500 is the number of wavelength spectral description slots.
   - a 'for' loop fills up the slots with 0
   - then the ffu.s array is filled with each of the spectral band data
     one after the other.
     Example Lsat TM: (435-250) / 2.5 = 74
 
 o add filter function to Iwave.cpp, before IWave::equivwl()
 o add else-if in IWave::parse() in Iwave.cpp
 o add print strings in Iwave.cpp
 
 o add iwave values and signature in Iwave.h
 o add to GeomCond.cpp and GeomCond.h
 o add to i.atcorr.html


Preparation of the Iwave.cpp file code:

The helper script create_iwave.py can help to interpolate the filter functions
to the correct step size of 2.5 nm (in case that the filter function data
are delivered with a different step size).
The output filter function in a format as needed in the Iwave.cpp file.

The script create_iwave.py basically needs a .csv file with spectral
response for each band in a column. The first column has to be the 
wavelength (nm). The first line (and only first) is a header with
wavelength, followed by band names file name is used for sensor name.
See for example ./sensors_csv/ikonos.csv.

Usage 
  create_iwave.py <csv file>

If generates the filter function as IWave.cpp template from csv file.

Note: If the spectral response is null for a frequency, leave it empty
in csv file. Ex.:

  WL(nm),band 1,band 2,band 3,band 4
  455,0.93,,,
  485,0.94,0.00,,
  545,0.00,0.87,0.00,

Program will interpolate filter function to 2.5 nm steps
and output a CPP template file in the IWave format.

