Purpose

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  This procedure swaps out NANs in an array with another value. It
  can also undo this swapping

Inputs

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  data: A data cube
  swap: A value to substitute in place of nans within data

Keyword Parameters

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  unswap: Reverse the procedure, replacing instances of swap with
          nan. Note that any naturally occuring instances of SWAP
          will be also replaced with nans.

Modification History

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  April 2010: Written by Chris Beaumont