seviri l2 grib add file names from Eumetsat datastore

[TAlonglong]

The seviri_l2_grib reader can read l2 grib files from the archive as far as I can understand.

With the eumetsat datastore a user can download the same data but with a slightly different filename.

Need to add this file name to be read.

Add after: https://github.com/pytroll/satpy/blob/master/satpy/etc/readers/seviri_l2_grib.yaml#L16 something like this:

- '{spacecraft:4s}-SEVI-MSGCLMK-0100-0100-{start_time:%Y%m%d%H%M%S}.000000000Z-NA.grb'

Also consider the other products in the reader if applicable.

[ameraner]

Hi @TAlonglong thanks for reporting this! @sjoro @ColinDuff FYI.

[ColinDuff]

This issue can be assigned to me