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 12707243706

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 78.283%

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Totals
Change from base Build 12688079447:	0.0%
Covered Lines:	1647
Relevant Lines:	2056

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💛 - 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]",
}