====== Handling of magnitude systems ======

The handling of magnitude systems was a bit confuse in version 2.0. We now use (for versions >=2.2) a more comprehensive scheme.
In ''fitmodel.card'', each magnitude system is identified by a key referencing a description file in the data directory. For instance, ''@VEGA MagSys/BD17-snls3.dat''
As for previous versions, the key has to be mentionned in the light curves, along with the instrument name and the pass band. For instance,

  @MAGSYS VEGA

The description file contains the path to a calibrated spectrum of a reference star ( in ergs/s/cm2/A), and a list of magnitudes for this star for various instruments and pass-bands. For instance, ''BD17-snls3.dat'' contains:
  # 2009 release of CALSPEC 
  @SPECTRUM MagSys/bd_17d4708_stisnic_002.ascii
  # Magnitudes in the Landolt system : Landolt & Uomoto (2007)
  STANDARD U 9.724
  STANDARD B 9.907
  STANDARD V 9.464
  STANDARD R 9.166
  STANDARD I 8.846
  
  # Magnitudes in the Megacam local natural system : Regnault et al (2009), Table 7
  MEGACAM u 9.7688 # annexe papier calib08
  MEGACAM g 9.6906
  MEGACAM r 9.2183
  MEGACAM i 8.9142
  MEGACAM z 8.7736


The labels for instruments has to be the same as in the fitmodel.card file:
''@STANDARD Instruments/SNLS3-Landolt-model''
The pass bands have to match those in the instrument filter wheel file.

In ''$PATHMODEL/Instruments/SNLS3-Landolt-model/instrument.cards''
  ...
  @FILTERS FilterWheel
  ...

In ''$PATHMODEL/Instruments/SNLS3-Landolt-model/FilterWheel''

  U U_bessel sux-shifted.dat
  B B_bessel sb-shifted.dat
  V V_bessel sv-shifted.dat
  R R_bessel sr-shifted.dat
  I I_bessel si-shifted.dat

The only exception is the AB magnitude system which directly implemented in the C++ code.