Visualize differences between time-independent model files

[dkang2]

Is your feature request related to a problem? Please describe.
Often times users want to investigate the differences in space for monthly or annually model simulations for different time periods. As right now VERDI can only handle files which have the same timestamp. If one tries to make difference of these files, VERDI would show

Describe the solution you'd like
As Chris Nolte pointed that this should be converted to a warning rather than an outright error, and/or there should be a way to override this check.
If the two files have the same number of time steps, with the same value for the time step, then the fact that the dates do not match should be ok. If the two files are time independent, with TSTEP=0, then definitely the program should allow one to compute differences.

[yadongxuEPA]

Tested with VERDI_2.1.5_linux64_20240524.tar.gz on Atmos, found that this issue has resolved.

Test 1: used these two data files
dataset 1: /work/MOD3DEV/dkj/Verdi/example_files/LTemis_MonthlyMean_GLM_202206.ncf
dataset 2: /work/MOD3DEV/dkj/Verdi/example_files/LTemis_MonthlyMean_GLM_202207.ncf
created this formula : NO[2]-NO[1]
display the tile plots and probed the values of the following grid cells in three tile plots.

checked and confirmed that the calculated values match.
grid cell NO[1] NO[2] NO[2]-NO[1]
(328,116) 0 0.054 0.054
(363,108) 0.067 0 -0.067
(347,104) 0.007 0.049 0.042

Test 2: used these two data files
dataset 1: /work/MOD3DEV/jherwehe/verdi/test/bkx/cmaq/wr401/task4/2018/cctm/extr/201808/combine_aconc_v53_intel18.0_2018ff_12US1_20180817
dataset 2:
/work/MOD3DEV/jherwehe/verdi/test/CMAQ_v53_Dev/CMAQv53b3fix_2016_12US1_M3Dry_Bidi/07/[CCTM_ACONC_v53_intel18.0_2016_CONUS_test_20160715.nc
created this formula : O3[2]-O3[1]
display the tile plots and probed the values of the following grid cells in three tile plots.

checked and confirmed that the calculated values match.

grid cell O3[1] O3[2] O3[2]-O3[1]
(383,193) 16.096ppbv 0.061ppm 45.255ppbv
(400,184) 72.435ppbv 0.027ppm -45.721ppbv
(362,179) 25.658ppbv 0.034ppm 8.041ppbv

Please note that in test 1, I did not receive any warning messages (maybe there are no timesteps in the datasets). In test 2, I received the following warning messages twice (once when add the new formula, then when do 'tile plot')