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  This function estimates the amount of reddening a star is seen
  through, based on its observed colors.


  mag: An NBAND by NOBJECT array, or an NBAND vector. Each row of the
      array represents an object, and the columns in that array list
      the magnitudes of that object in NBAND different photometric
      bands. If NBAND = 3, the colors are assumed to be (J,H,Ks). If
      NBAND = 5, the colors are assumed to be (J,H,Ks,I1,I2). In
      general, any set of magnitudes is allowed, as long as the
      appropriate keyword variables are passed (see below).
  dmag: The uncertainty on each magnitude measurement.

Keyword Parameters

  color: A vector of the mean color of (unreddened) stars. An NBAND-1
        vector. Each entry refers to the mean color formed by taking
        mag_i - mag_i+1. For example, if NBAND = 3, the assumed
        magnitudes are JHK, and the colors are J-H and H-K. Default
        values are provided for (JHK) and (JHK I1 I2) if NBAND = 3
        or 5. Otherwise, this vector must be provided.
  covar: An NBAND-1 by NBAND-1 matrix, specifying the covariance of
        colors of unreddened stars. This is provided for (JHK) and
        (JHK I1 I2) if NBAND = 3 or 5. Otherwise, this matrix must
        be provided.
    ks: The reddening vector for each color. That is,
        c_i(observed) = c_i(true) + Av * k. This is provided for
        (JHK) and (JHK I1 I2) when NBAND = 3 or 5. Otherwise, this
        vector must be provided.


  Algorithm taken from Lombardi and Alves 2001. Equation references
  in code refer to that paper.
  JHK extinction law taken from Lebofsky 1985
  IRAC extinction law taken from Indebetouw, Mathis et al. 2005



Modification History

  Written by: Chris Beaumont July 2008
  November 2008: cnb. Updated to incorporate IRAC bands 1 and 2
  November 2009: cnb. Completely re-written to better generalize to
                      different photometric bands. Calling sequence changed.

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