Issue 538: Skip individual split files that already exist

[mmarusiak]

Fixes #538

Previously, all three splitters used an all-or-nothing approach: if any output file existed (via wildcard match), the entire input file was skipped. If even one output file was missing, the splitter re-split the whole input and overwrote all previously created children.

Now each splitter checks existence per output file and only generates the missing ones:

• GribSplitter: replaced the upfront should_skip() guard with a per-key should_skip_file() check inside the message loop, using a skipped_keys set to avoid redundant filesystem calls for the same output path.
• GribSplitterV2: already checked per-file before running grib_copy, but the upload loop was copying every file grib_copy produced (including already-existing ones). Fixed by filtering the upload loop to only paths in output_paths.
• NetCdfSplitter: removed the upfront should_skip() guard and moved the existence check into _write_dataset(), which now returns a bool so split_data() can track how many files were actually written.

Added test_splits_only_missing_files for both GribSplitter/GribSplitterV2 (parametrized) and NetCdfSplitter: full split, delete one output file, re-split, assert only the deleted file was recreated and all others have unchanged modification times.