Memory efficient output of ModvegeSite when parameter-scanning

When running parameter scans for sites with data of many years, a large number of `ModvegeSite` objects arise as output.
Storing all of them can lead to file sizes in the hundreds of MB.

Since we anyways only require very limited information, extract that data instead of storing the full `ModvegeSite` objects.

Currently, the required fields are:
- `dBM`
- `cBM`

