Add vis/ir cloud composite for FCI

[gerritholl]

First attempt of a VIS/IR cloud composite for FCI, based on the CLM product from EUMETSAT.

Usage example:

import hdf5plugin
from satpy import Scene
from glob import glob
import os
sc = Scene(
        filenames=
            {"fci_l2_nc": ["W_XX-EUMETSAT-Darmstadt,IMG+SAT,MTI1+FCI-2-CT--FD------NC4E_C_EUMT_20260105164431_L2PF_OPE_20260105163000_20260105164000_N__C_0100_0000.nc"],
             "fci_l1c_nc": glob("*FDHSI*BODY*20260105*O_0100_*.nc")})
sc.load(["vis_06", "ir_105", "_vis_with_ir",
         "_vis_06_crude", "_ir105", "vis_with_ir_cloud_overlay"])
ls = sc.resample("eurol", resampler="nearest", radius_of_influence=20000) ls.save_datasets(
        filename="{platform_name}-{sensor}-{area.area_id}-{name}-{start_time:%Y%m%d%H%M%S}-{end_time:%Y%m%d%H%M%S}.tif",
        fill_value=0)

Result for _vis-with-ir (no overlay) for 2026-01-05 16:30:

Result for vis_with_ir_cloud_overlay:

NB: Transparency may show as black or white depending on the background, or when viewed in a client not supporting transparency. This is how irfanview shows it:

• Closes #xxxx
• Tests added
• Fully documented

[gerritholl]

Example with a background layer to illustrate the transparency better:

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.34%. Comparing base (4597879) to head (3604a54).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3325   +/-   ##
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  Coverage   96.34%   96.34%           
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  Files         463      463           
  Lines       58970    58970           
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  Hits        56813    56813           
  Misses       2157     2157           

Flag	Coverage Δ
behaviourtests	3.59% <ø> (ø)
unittests	96.43% <ø> (ø)

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[coveralls]

Pull Request Test Coverage Report for Build 21817250891

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 96.422%

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Totals
Change from base Build 21752490617:	0.0%
Covered Lines:	56693
Relevant Lines:	58797

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