Made the function getSag more flexible for absorbance dataframes that do...

Created by: ghost

... not have the required wavelengths defined in fxn input 'sag.' The fxn finds the nearest wavelength (defined in sag) in the input absorbance dataframe instead. The summary variable name is SagXXX_XXX is changed accordingly.ALSO- in the case where all lines 51-59 were created to deal with the case where all aCorr observations are < 0