Add Himawari-8 AHI L2 reader

[zxdawn]

This PR adds a reader for the Himawari-8 AHI L2 data which are NC files.

• Tests added
• Tests passed
• Passes flake8 satpy
• Fully documented

[zxdawn]

Current stage of this reader:

1. No end_time property

As only start_time is shown in the filename and many files miss end_time, I don't assign values to it.

2. csv not supported

There's a special L2 data called Wildfire. Here's the filename pattern:

 H08_YYYYMMDD_hhmm_L2WLFVER_FLDK.xxxxx_yyyyy.csv

 where YYYY: 4-digit year of observation start time (timeline);
       MM: 2-digit month of timeline;
       DD: 2-digit day of timeline;
       hh: 2-digit hour of timeline;
       mm: 2-digit minutes of timeline;
       VER: version;
       xxxxx: pixel number; and
       yyyyy: line number.

 Example: H08_20150727_0800_L2WLFbet_FLDK.06001_06001.csv

Maybe we can add another reader called ahi_l2_csv. Any suggestions for this problem?

[coveralls]

Coverage decreased (-0.1%) to 89.882% when pulling b65d533 on zxdawn:h8_l2 into b909413 on pytroll:master.

[simonrp84]

Where is this Himawari L2 data from? I guess p-tree?
The file name format is different to the data on the AWS cloud (e.g: https://noaa-himawari8.s3.amazonaws.com/index.html#AHI-L2-FLDK-Clouds/2020/06/04/0030/) so I wonder if the internal data in the netCDF is also different. Hopefully not!

This is mainly just a note to remind me to look into this when I have some free time later in the week, it'd be good to support both source of L2 data.

[codecov]

Codecov Report

Merging #1250 into master will decrease coverage by 0.14%.
The diff coverage is 24.28%.

@@            Coverage Diff             @@
##           master    #1250      +/-   ##
==========================================
- Coverage   90.02%   89.88%   -0.15%     
==========================================
  Files         218      219       +1     
  Lines       31269    31339      +70     
==========================================
+ Hits        28151    28168      +17     
- Misses       3118     3171      +53     

Impacted Files	Coverage Δ
satpy/readers/ahi_l2_nc.py	24.28% <24.28%> (ø)

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[zxdawn]

@simonrp84 Yes, they're from p-tree. The attached README txt illustrates all information. BTW, it seems the L2 data on AWS just has three cloud products, right? Then, that's different from these data on p-tree ...

Not sure that we need two readers or just one for dealing with both kinds of data.

README_HimawariGeo_en.txt

[simonrp84]

@zxdawn Hmm, that looks quite different. I'll investigate more later in the week!

[songhan89]

There's also another set of L2 product from JMA in .grib. I've uploaded some samples here.

Not sure if you would like to incorporate them.

Sample files for JMA HCAI

• https://www.data.jma.go.jp/mscweb/en/product/product_HCAI.html
• HCAI Browser

[djhoese]

Any GRIB files should be handled by a separate reader. Either the generic grib reader or a new ahi_l2_grib reader.

[mraspaud]

Great work so far @zxdawn ! don't hesitate to put some image previews here to, it's always nice to see what it will look like.

[zxdawn]

@mraspaud Thanks! I'm writing a parallax_correction composite based on Cloud Top Height as suggested by @simonrp84. I would post some CTH images here and create a PR for the composite to check the effect of correction ;)

(I'm preparing the proposal for my PhD research, so that would be a little later)