PlasmaConstant:

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   Constants useful in plasma physics.
   If no inputs are entered it prints the list in the catalog.

   Type PlasmaConstant to get a catalog of constants.
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   Form:
   [x, u] = PlasmaConstant( q )
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   Inputs
   ------
   q          (1,:)	String describing the constant

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   Outputs
   -------
   x          (1,1)	The value
   u          (1,:)	Units (if applicable)

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