CORRECT_FOR_ATTENUATION algorithm

[NickDraper]

This correction depends on sample geometry, angle and wavelength, and should be ultimately done point by point. It is common to essentially all ISIS instruments dealing with bulk samples, and should be the subject of a common project. For the purpose of this exercise, we can use the ARIEL approach which is that of calculating a polynomial approximation for each group (the variable is wavelength). Then you apply it to the focussed data (in d-spacing) by using the average 2theta of the bank for the conversion. I can initially supply the pre-calculated polynomials just for test, and I have fairly simple routines to calculate them that can be easily ported.

[NickDraper]

This issue was originally trac ticket 228