Fix masking of AHI HSD space pixels

[sfinkens]

Replace usage of satpy.readers.ahi_hsd.AHIHSDFileHandler.geo_mask with satpy.readers.utils.get_geostationary_mask because they do the same thing.

A closer analysis revealed that the masks computed by the two methods are shifted vertically by one pixel. The reason for that are incorrect line indices in geo_mask (shifted by one). If applying the following modification, both methods compute exactly the same masks. Therefore it should be safe to replace geo_mask with get_geostationary_mask.

diff --git a/satpy/readers/ahi_hsd.py b/satpy/readers/ahi_hsd.py
index 538b6af9..43fb70e9 100644
--- a/satpy/readers/ahi_hsd.py
+++ b/satpy/readers/ahi_hsd.py
@@ -341,8 +341,8 @@ class AHIHSDFileHandler(BaseFileHandler):
         cols_idx = da.arange(-(coff - local_coff),
                              ncols - (coff - local_coff),
                              dtype=np.float, chunks=CHUNK_SIZE)
-        lines_idx = da.arange(nlines - (loff - local_loff),
-                              -(loff - local_loff),
+        lines_idx = da.arange(nlines - 1 - (loff - local_loff),
+                              -(loff - local_loff) - 1,
                               -1,
                               dtype=np.float, chunks=CHUNK_SIZE)
         return ellipse(lines_idx[:, None], cols_idx[None, :])

• Closes Mask space pixels in AHI HSD reader #531
• Tests added
• Tests passed
• Passes git diff origin/master -- "*py" | flake8 --diff

[djhoese]

LGTM. I think I'm going to mark this as a "bug fix" for 0.11.2 and will merge right away. Technically this is a "feature" or at least an optimization but given the off by one issue that was discovered this sounds more like a bug fix. Correct me if I'm wrong.

[sfinkens]

@djhoese Agree!

[coveralls]

Coverage increased (+0.1%) to 77.566% when pulling 9aec74e on sfinkens:fix_531 into 73c1600 on pytroll:master.

[codecov]

Codecov Report

Merging #592 into master will increase coverage by 0.19%.
The diff coverage is 36.11%.

@@            Coverage Diff             @@
##           master     #592      +/-   ##
==========================================
+ Coverage   77.45%   77.64%   +0.19%     
==========================================
  Files         136      136              
  Lines       19168    19248      +80     
==========================================
+ Hits        14846    14946     +100     
+ Misses       4322     4302      -20

Impacted Files	Coverage Δ
satpy/tests/reader_tests/test_ahi_hsd.py	96.51% <100%> (+0.79%)	⬆️
satpy/readers/ahi_hsd.py	72.98% <17.85%> (+11.99%)	⬆️
satpy/multiscene.py	83.46% <0%> (+4.34%)	⬆️

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