Add CI check for number of grid files in gridpack

Changes introduced

• New check implemented in check_grid (not skippable) which counts how many files a gridpack contains and throws an error if nfiles > 80k
• Renaming of grid-related CI jobs (for clarity)

Tests

Gridpack with more than 80k files

Gridpack with less than 80k files

Issues resolved

Closes #117 (closed)

I have to use [skip all] since the renamed files under scripts are not in the whitelist (we don't want people to put new scripts there).