Update AHI HSD reader to correctly handle singleton arrays.

[simonrp84]

Recently, one of the libraries used by satpy (I think numpy) has started warning about conversion of arrays to scalar:

C:\Users\Simon\PycharmProjects\satpy\satpy\readers\ahi_hsd.py:422: DeprecationWarning: Conversion of an array with ndim > 0 to a scalar is deprecated, and will error in future. Ensure you extract a single element from your array before performing this operation. (Deprecated NumPy 1.25.)
  return datetime(1858, 11, 17) + timedelta(days=float(self.basic_info["observation_start_time"]))

This PR updates the ahi_hsd reader and tests to remove the warning by selecting the first element of the relevant array. As the array only has one element, this means there's no change in any results on the user-side.

• Tests added

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f4725c2) 95.32% compared to head (3f55256) 95.32%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2682   +/-   ##
=======================================
  Coverage   95.32%   95.32%           
=======================================
  Files         371      371           
  Lines       52441    52441           
=======================================
  Hits        49988    49988           
  Misses       2453     2453           

Flag	Coverage Δ
behaviourtests	4.16% <0.00%> (ø)
unittests	95.93% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

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

[coveralls]

Pull Request Test Coverage Report for Build 7183035793

• 17 of 17 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 95.886%

---

Totals
Change from base Build 7180375207:	0.0%
Covered Lines:	50114
Relevant Lines:	52264

---

💛 - Coveralls

[sfinkens]

Nice work, thanks a lot!

[mraspaud]

LGTM!

[simonrp84]

Ooops, I messed up the tests. Should be fixed now.