Formally add extrap_x as a Spectrum attribute.
Formally add extrap_x as a Spectrum attribute.

Primarily, this is to ensure it gets properly passed when a Spectrum is manipulated arithmetically. Note that if two Spectra with different extrap_x are combined arithmetically, the result will have extrap_x = None.



git-svn-id: 4c7b13231a96299fde701bb5dec4bd2aaf383fc6@363 979d6bd5-6d4d-0410-bece-f567c23bd345

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