Use spline interpolation for faster processing

[mraspaud]

This PR makes use of the newer spline interpolation in python geotiepoints to improve the performance of the SAR and SGLI readers.

This requires pytroll/python-geotiepoints#85 to be merged and released.

• Tests added

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.07%. Comparing base (7eb17ab) to head (12ebe3f).
Report is 48 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2927   +/-   ##
=======================================
  Coverage   96.07%   96.07%           
=======================================
  Files         373      373           
  Lines       54479    54491   +12     
=======================================
+ Hits        52338    52352   +14     
+ Misses       2141     2139    -2     

Flag	Coverage Δ
behaviourtests	3.99% <0.00%> (-0.01%)	⬇️
unittests	96.17% <100.00%> (+<0.01%)	⬆️

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[coveralls]

Pull Request Test Coverage Report for Build 11353495088

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 14 of 14 (100.0%) changed or added relevant lines in 3 files are covered.
• 6 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.004%) to 96.17%

---

Files with Coverage Reduction	New Missed Lines	%
satpy/composites/init.py	6	95.05%

Totals
Change from base Build 11304823079:	0.004%
Covered Lines:	52586
Relevant Lines:	54680

---

💛 - Coveralls

[pnuu]

LGTM, just one comment inline.