Add satellite position to dataset attributes (seviri_l1b_hrit)

[sfinkens]

The seviri_l1b_hrit reader currenlty only provides the longitude and latitude of the projection centre. However, the projection centre does not necessarily represent the actual position of the satellite. For some applications the actual position is required. From the MSG Level 1.5 Image Data Format Description:

In practise, if the user needs (e.g.) to find a location of a pixel on the Earth ellipsoid, he/she needs to refer to the image projection. If one needs to refer to (e.g) the satellite viewing angles, one needs to refer to the satellite position.

This PR adds the nominal and actual satellite position to the dataset attributes:

• Nominal & actual satellite position can be found under navigation.
• Projection centre and altitude can be found under projection.
• Existing keys satellite_longitude/latitude/altitude remain
  unchanged for the moment

Example:

from satpy import Scene
import glob

filenames = glob.glob('data/*200601011215*')
scene = Scene(filenames, reader='seviri_l1b_hrit')
scene.load(['IR_108'])

print(scene['IR_108'].attrs['projection'])
print(scene['IR_108'].attrs['navigation'])

Output:

{'satellite_longitude': 0.0, 'satellite_latitude': 0.0, 'satellite_altitude': 35785831.0}
{'satellite_nominal_longitude': -3.4, 'satellite_nominal_latitude': 0.0, 'satellite_actual_longitude': -3.55117540521326, 'satellite_actual_latitude': -0.5705405815406472, 'satellite_actual_altitude': 35783328.146167256}

• Addresses Refine Satellite Position #670
• Tests added
• Tests passed
• Passes git diff origin/master -- "*py" | flake8 --diff

[coveralls]

Coverage increased (+0.2%) to 78.837% when pulling fbeb223 on sfinkens:feature-seviri-hrit-satpos into 40a0dcd on pytroll:master.

[codecov]

Codecov Report

Merging #671 into master will increase coverage by 0.17%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #671      +/-   ##
==========================================
+ Coverage   78.66%   78.84%   +0.17%     
==========================================
  Files         138      138              
  Lines       20356    20479     +123     
==========================================
+ Hits        16013    16146     +133     
+ Misses       4343     4333      -10

Impacted Files	Coverage Δ
satpy/readers/seviri_base.py	97.26% <100%> (+0.11%)	⬆️
satpy/readers/seviri_l1b_hrit.py	76.66% <100%> (+12.3%)	⬆️
satpy/tests/reader_tests/test_seviri_base.py	97.05% <100%> (+2.05%)	⬆️
satpy/tests/reader_tests/test_seviri_l1b_hrit.py	97.87% <100%> (+0.81%)	⬆️
satpy/readers/hrit_base.py	80.86% <100%> (+0.11%)	⬆️
satpy/readers/avhrr_l1b_gaclac.py	8.33% <0%> (-0.18%)	⬇️
satpy/writers/geotiff.py	42.01% <0%> (ø)	⬆️

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[mraspaud]

LGTM

[mraspaud]

@djhoese are you ok adding pyproj as a dependency ?