BPnd (Ichise MRTM2 Ref): Ichise MRTM with fixed k2'

MRTM can be turned into a more robust method called MRTM2 for pixel-wise applications with the same two-step approach applied in the SRTM2 model:

1. Calculate in the model preprocessing step the clearance rate k₂' of the reference TAC by the MRTM method with VOI data which has a limited level of noise.
2. Fix k₂': Use the estimated k₂' value for the pixel-wise MRTM calculations, reducing the number of fitted parameters from 3 to 2.

If k'₂ is fixed, the equation of MRTM reduces to the MRTM2 equation below

with only two regression coefficients V/(V'b) and 1/b for T > t*. BP is calculated from the ratio of the two regression coefficients as