Finish round 1 of Darwin Core mapping

Things done:

1. Create 3 files for mapping: `change-fields`, `add-fields` and `delete-fields` (commit 98bbb7b)
2. Move all fields from `obis-to-dwc-mapping` to one of these 3 files and remove original file (done in commit 9c5a906)
3. Review **all terms** in `dwc-mapping` and indicate status with `-` (don't use) or `suggested` (suggest to use). (finished in commit 9268a6e)

This finishes our round 1 of Darwin Core mapping. Now we need feedback from the Diveboard team, to see which suggested terms can actually be used, and which ones cannot.

@DimEvil, can you review my commits, in particular the ones with `Don't use` which often have extra explanation why I recommend **not** to use a term? If all looks good, you can merge this request.