Fmi geotiff importer fixes

[pulkkins]

This pull request contains the following fixes to the FMI GeoTIFF importer (io.importers.import_fmi_geotiff):

• remove unnecessary UTF-8 encoding lines
• import all gdal modules from osgeo instead of the global namespace (required by GDAL>=3.2)
• remove the GetScale() or GetOffset() lines because these are hard-coded in the files
• set the Z(R) relation parameters in the metadata to zr_a=223 and zr_b=1.53
• instead of None, set the unit to dBZ in the metadata
• set transform to 'dB' in the metadata
• add separate section for FMI GeoTIFF files in pystepsrc
• add tests for the FMI GeoTIFF importer
• add gdal as optional dependency in the user guide
• update black to version 22.3.0 in .pre-commit-config.yaml

[codecov[bot]]

Codecov Report

Merging #284 (c07b601) into master (f3216b2) will increase coverage by 0.26%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #284      +/-   ##
==========================================
+ Coverage   82.39%   82.66%   +0.26%     
==========================================
  Files         158      159       +1     
  Lines       12165    12180      +15     
==========================================
+ Hits        10023    10068      +45     
+ Misses       2142     2112      -30     

Flag	Coverage Δ
unit_tests	82.66% <100.00%> (+0.26%)	:arrow_up:

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Impacted Files	Coverage Δ
pysteps/tests/test_io_fmi_pgm.py	100.00% <ø> (ø)
pysteps/io/importers.py	76.37% <100.00%> (+4.34%)	:arrow_up:
pysteps/tests/test_io_fmi_geotiff.py	100.00% <100.00%> (ø)
pysteps/io/exporters.py	54.84% <0.00%> (-0.28%)	:arrow_down:

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[dnerini]

Very good, thanks, you can go ahead and merge into master once all tests are green.