Calculation for thickness in da_predictor_rttov

[IanhSi]

The 1000-300hPa thickness is calculated as follows in WRFDA:
code1: pred(1) = kth * sum(tv(index300+1:index1000) * dlp(index300+1:index1000)) + add_thk.

However, I found there might be an index error in the array, causing misalignment. The correct code should be
code2: pred(1) = kth * sum(tv(index300:index1000-1) * dlp(index300:index1000-1)) + add_thk.

For validation, I used MetPy's metpy.calc.thickness_hydrostatic, and the calculated values for 1000-300 thickness in MetPy, code1 and code2 are 9586.05, 9270.26, and 9581.17, respectively.

[weiwangncar]

@liujake Can you take a look at this? Thanks.