Fix SLSTR angle interpolation.

[simonrp84]

This PR adds a new method for interpolating angles from the SLSTR tie grid, which should prevent the unrealistically low and/or high values that sometimes showed up before, primarily in granules near the poles.

• Closes extrapolation of satellite relative azimuiths? #3015
• Fully documented

[codecov]

Codecov Report

Attention: Patch coverage is 27.27273% with 8 lines in your changes missing coverage. Please review.

Project coverage is 96.08%. Comparing base (5984c29) to head (8bb668a).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
satpy/readers/slstr_l1b.py	27.27%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3017      +/-   ##
==========================================
- Coverage   96.10%   96.08%   -0.02%     
==========================================
  Files         377      377              
  Lines       55163    55172       +9     
==========================================
+ Hits        53012    53013       +1     
- Misses       2151     2159       +8     

Flag	Coverage Δ
behaviourtests	3.94% <0.00%> (-0.01%)	⬇️
unittests	96.18% <27.27%> (-0.02%)	⬇️

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 12322983456

Details

• 3 of 11 (27.27%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.01%) to 96.193%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
satpy/readers/slstr_l1b.py	3	11	27.27%

Totals
Change from base Build 12299617024:	-0.01%
Covered Lines:	53257
Relevant Lines:	55365

---

💛 - Coveralls