Fix xarray to grib from temporal accumulation

[sandorkertesz]

Description

This PR fixes the issue when Xarray generated from GRIB data containing temporal accumulation was not correctly written back to GRIB.

The problem can be demonstrated with this code:

import earthkit.data as ekd

input_dir = "... /cama_step1/"
output_dir = "./"
 
year_str="1979"

# Load dataset
ds_snow = ekd.from_source("file", f"{input_dir}iqtn_{year_str}.grib")
   
# Convert to xarray
ds_snow_xr = ds_snow.to_xarray(time_dim_mode="valid_time")
  
# Write back to GRIB
ds_snow_xr_daily.earthkit.to_grib( f"{output_dir}snow_daily_{year_str}.grib")

It produces a GRIB file with incorrect time. E.g. in message 20 we see:

dataDate: 19790120
dataTime: 0
endSte:6
validityDate: 19790101
validityTime: 600

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.72%. Comparing base (1733b6f) to head (7f91337).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #799      +/-   ##
===========================================
+ Coverage    85.68%   85.72%   +0.03%     
===========================================
  Files          172      172              
  Lines        13193    13222      +29     
  Branches       637      640       +3     
===========================================
+ Hits         11305    11334      +29     
  Misses        1698     1698              
  Partials       190      190              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.