Fix MTD to compute the CDIST_TRAVELLED value correctly.

[CPKalb]

Describe the Problem

In the calculation of cdist_travelled, x_old and y_old are set in the first loop iteration as the values of xbar_2d and ybar_2d. However, these values don't change in successive iterations of the loop. This results in the distance being calculated using the starting centroid lat/lon rather than the centroid lat/lon of the previous time step, giving a cdist_travelled that is too large.

Expected Behavior

x_old and y_old should use the values of xbar_2d and ybar_2d from the previous loop iteration.

Environment

Describe your runtime environment:

1. Machine: Linux Workstation (aerial)
2. OS: Debian GNU/Linux 9.13 (stretch)
3. Lines 1319 - 1336, https://github.com/dtcenter/MET/blob/main_v10.0/met/src/tools/other/mode_time_domain/3d_att.cc

To Reproduce

1. Run mtd (mtd -single mtd_file_list_test.txt -config MTDConfig_SKM_python_ALP3i_MOHC_HadREM3_rcp85_prltTH05_rsmoo8_tsmoo1_minvol1000_oooo)
2. Object F001 has a computed cdist_travelled of 354.691
3. Compute the great circle distance between successive centroid lat/lons, using mtd_ALP-3i_MOHC_HadREM3_rcp85_prltTH05_rsmoo8_tsmoo1_minvol1000_oooo__20960314_000000V_2d.txt . The value computed externally is approximately 252 (using a constant earth radius of 6371)

Post relevant sample data following these instructions:*
The sample data is on seneca in /d1/projects/METplus/discussions/1249

Relevant Deadlines

MET 10.1.0-beta5

Funding Source

2702691

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