Function to generate Bland Altman plots. Barry Greene, September 2008
Bland, J.M., Altman, D.G. 'Statistical methods for assessing agreement ...
between two methods of clinical measurement'(1986) Lancet, 1 (8476), pp. 307-310.
Inputs: data1: Data from first instrument
data2: Data from second instument
Outputs: data_mean: Mean of data recorded by both instruments
data_diff: Difference of data recorded by both instruments
md: Mean difference
sd: Standard deviation of difference
Produces Bland Altman plot with mean difference and mean difference +/-
2*SD difference lines.