start-period and end-period are sometimes combined in a single field

examples are:

```
1881 until today
1921 - 2020
1930 until today
1945-2020
1947-1978
1947-1981
1947-1991
1949-1972 and 2004-2014
1950 - 2025
```

for those cases where we can easily deduce them as 2 elements begin-end, we can split them to the 2 relevant fields

this may also be a problem of the harvester, maybe a harvester does not correctly distinguish them
