Add the check_satpy function to find missing dependencies

[mraspaud]

This PR add a check_satpy function to diagnose the satpy installation and flag possible dependency problems with readers and writers

An example of the output of the function is:

Readers
=======
abi_l1b: ok
acspo: ok
ahi_hsd: ok
amsr2_l1b: ok
avhrr_aapp_l1b: ok
avhrr_eps_l1b: ok
avhrr_hrpt_l1b: cannot find module 'satpy.readers.hrpt' (No module named 'pygac')
clavrx: ok
fci_fdhsi: ok
gac_lac_l1: cannot find module 'satpy.readers.gac_lac_l1' (No module named 'pygac')
generic_image: ok
geocat: ok
ghrsst_osisaf: ok
grib: cannot find module 'satpy.readers.grib' (No module named 'pygrib')
hdf4_caliopv3: ok
hdfeos_l1b: ok
hrit_electrol: ok
hrit_goes: ok
hrit_jma: ok
hrit_msg: ok
iasi_l2: ok
li_l2: ok
maia: ok
native_msg: ok
nc_goes: ok
nc_nwcsaf_msg: ok
nc_nwcsaf_pps: ok
nc_olci_l1b: ok
nc_olci_l2: ok
nc_slstr: ok
nucaps: ok
omps_edr: ok
safe_msi: ok
safe_sar_c: cannot find module 'satpy.readers.safe_sar_c' (No module named 'osgeo')
scatsat1_l2b: ok
scmi_abi_l1b: ok
viirs_compact: ok
viirs_edr_flood: ok
viirs_l1b: ok
viirs_sdr: ok

Writers
=======
cf: ok
geotiff: cannot find module 'satpy.writers.geotiff' (No module named 'osgeo')
mitiff: ok
ninjotiff: cannot find module 'satpy.writers.ninjotiff' (No module named 'pyninjotiff')
scmi: ok
simple_image: ok

• Tests added
• Tests passed
• Passes git diff origin/master -- "*py" | flake8 --diff
• Fully documented

[djhoese]

This makes me think of vispy's sys_info() function which prints out versions of vispy, other dependency packages, and current OpenGL version + OpenGL extensions. Maybe this should do something similar at the top if numpy, pyresample, trollimage, pykdtree, python version, etc. Then we can have people include that in their issues.

[coveralls]

Coverage increased (+0.2%) to 74.109% when pulling e33de6b on mraspaud:feature-check-satpy into 5dd28ea on pytroll:master.

[codecov]

Codecov Report

Merging #488 into master will increase coverage by 0.19%.
The diff coverage is 21.42%.

@@            Coverage Diff             @@
##           master     #488      +/-   ##
==========================================
+ Coverage   73.93%   74.13%   +0.19%     
==========================================
  Files         137      137              
  Lines       18222    18257      +35     
==========================================
+ Hits        13472    13534      +62     
+ Misses       4750     4723      -27

Impacted Files	Coverage Δ
satpy/tests/test_writers.py	97.31% <100%> (+0.11%)	⬆️
satpy/config.py	46.66% <15.38%> (-24.49%)	⬇️
satpy/scene.py	84.03% <0%> (-0.43%)	⬇️
satpy/tests/test_scene.py	99.32% <0%> (-0.01%)	⬇️
satpy/composites/viirs.py	84.09% <0%> (ø)	⬆️
satpy/tests/test_enhancements.py	98.55% <0%> (ø)	⬆️
satpy/readers/hrit_msg.py	44.63% <0%> (ø)	⬆️
satpy/writers/geotiff.py	43.51% <0%> (ø)	⬆️
satpy/readers/abi_l1b.py	96.66% <0%> (+0.02%)	⬆️
satpy/readers/iasi_l2.py	98.8% <0%> (+0.02%)	⬆️
... and 10 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5dd28ea...e33de6b. Read the comment docs.