Add a median filter modifier

[mraspaud]

This PR adds a median filter modifier to allow de-noising.

• Tests added
• Fully documented

Example usage:

  median_filtered_chl_nn:
    standard_name: chl_nn
    compositor: !!python/name:satpy.composites.SingleBandCompositor
    prerequisites:
      - name: chl_nn
        modifiers:
          - median5x5

[codecov]

Codecov Report

Merging #2521 (d22e117) into main (bcd6581) will decrease coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #2521      +/-   ##
==========================================
- Coverage   94.96%   94.91%   -0.06%     
==========================================
  Files         347      349       +2     
  Lines       50921    50969      +48     
==========================================
+ Hits        48358    48375      +17     
- Misses       2563     2594      +31     

Flag	Coverage Δ
behaviourtests	4.31% <0.00%> (-0.01%)	⬇️
unittests	95.51% <100.00%> (-0.06%)	⬇️

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

Impacted Files	Coverage Δ
satpy/modifiers/filters.py	100.00% <100.00%> (ø)
satpy/tests/modifier_tests/test_filters.py	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

[coveralls]

Pull Request Test Coverage Report for Build 5399683404

• 38 of 38 (100.0%) changed or added relevant lines in 2 files are covered.
• 34 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.06%) to 95.464%

---

Files with Coverage Reduction	New Missed Lines	%
satpy/writers/cf_writer.py	1	94.9%
satpy/tests/writer_tests/test_cf.py	33	95.53%

Totals
Change from base Build 5389825507:	-0.06%
Covered Lines:	48488
Relevant Lines:	50792

---

💛 - Coveralls

[djhoese]

Looks good to me.