Simplify the creation of a CustomInstrument from response matrice

[sguillot]

The max/min input and max/min channels may be difficult to adjust as the choice of values can raise errors if some rows or columns of the matrix are filled with zero (or negative) values. By itself this is not a problem and this was implemented to save computing time.

The current error message tells the user to reduce the channels ranges to avoid this issue, without explaining the reason behind it. This could be clarified.

In practice, it shouldn't be necessary for the user to specify any range of max/min input and max/min channels. The full matrix should be loaded, and then only the part of the matrix relevant to the input data should be use for the calculation.

I think this could be transparent to the user (as it is currently for all X-ray spectral analysis packages), and therefore less complicated for the creation of the CustomInstrument, with perhaps some messages displayed to indicate the range of channels used.

Such improvement could be implemented at the same time as the FITS file implementation of the response matrices (#194)