Add SourceField and subclasses, rework Source
#1360
| Job | Run time |
|---|---|
| 6m 45s | |
| 2m 1s | |
| 1m 49s | |
| 2m 30s | |
| 2m 3s | |
| 1m 31s | |
| 2m 27s | |
| 2m 9s | |
| 1m 24s | |
| 1m 48s | |
| 1m 46s | |
| 1m 46s | |
| 1m 53s | |
| 3m 17s | |
| 4m 42s | |
| 3m 15s | |
| 3m 16s | |
| 4m 51s | |
| 49m 13s |