Update shared coast IDs to EEZ MRGID

[brynpickering]

Currently, shared-coast.csv is built based on EEZ 'ids', which are based on the geojson feature numbers generated by fiona when reading the shapefile. These IDs are not actually in the dataset itself, instead we should use mrgid, which is a unique identifier for each EEZ shape.

This update is required for calliope-project/euro-calliope#99 to function correctly, since automatic download of EEZs can only be done with v11 (currently, we use v10), which has a different order of shapes, effectively messing up the use of feature IDs as an identifier.