better dependecy handling

[BengtRydberg]

This PR improves the dependency handling.

If you install the package by pip install . only some default packages will be installed needed for the most important script of the level1c4pps package.

If you install it by pip install .[extra] you will install some additional packages needed for some specific satpy readers.

I am not totally sure which readers that should installed by default, so comments are welcome.

Note that level1c4pps utilises more readers than it looks like in the modified setup.py file as we only need to specify the readers that requires a non default satpy installation.

[coveralls]

Pull Request Test Coverage Report for Build 13053826588

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 21 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.1%) to 78.146%

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Files with Coverage Reduction	New Missed Lines	%
level1c4pps/tests/test_viirs2pps.py	2	84.21%
level1c4pps/init.py	7	85.78%
level1c4pps/viirs2pps_lib.py	12	48.15%

Totals
Change from base Build 12688079447:	-0.1%
Covered Lines:	1648
Relevant Lines:	2061

---

💛 - Coveralls

[BengtRydberg]

note that it is possible to separate this further if desired, e.g.:

extras_requires = {
     "gaclac": "satpy[avhrr_l1b_gaclac]",
     "seviri": "satpy[seviri_l1b_hrit,seviri_l1b_native,seviri_l1b_nc]",
     "metimage": "satpy[vii_l1b_nc]",
}

[ninahakansson]

I think the further specification is good.

[BengtRydberg]

I added more options.