Issue #997 regridders weights cache remove args

[JoerivanEngelen]

Fixes #997

Description

This PR removes the arguments source and target from RegridderWeightsCache.__init__, as they were not used internally.

Checklist

• [x] Links to correct issue
• [x] Update changelog, if changes affect users
• [x] PR title starts with Issue #nr, e.g. Issue #737
• [ ] Unit tests were added
• [ ] If feature added: Added/extended example