Calculation mode difference fit is incorrect because CarbonXS does not load an input pattern

[lktsui]

CarbonXS does not read in a dataset when running in calculation mode, so the output carbon.dat file does not include a set of source values to compare against.

Furthermore, the x-values may not match the currently loaded source file.

We should proceed as follows:

• Read the x-fit, y-fit values from carbon.dat
• Interpolate the y-fit values to the x-source values using linear interpolation
• Subtract using the interpolated values