Reverse frequency sweeping on Rohde&Schwarz ZNB40 via QCoDeS

Hi,

This is mainly a feature request but it would be nice if the user has the option to define a frequency sweep in the opposite direction for a measurement channel. Currently for instance, if measuring S21 for a DUT, we set

```python
from qcodes.instrument_drivers.rohde_schwarz.ZNB import ZNB
station.add_component(
ZNB(
name="VNA",
address="",
init_s_params=True,
)
)
station.VNA.S21.frequency_start(500e6)
station.VNA.S21.frequency_stop(1000e6)
```
to perform the measurement. However, QCoDeS gives out an error when setting the attribute `frequency_stop` to be lesser than `frequency_start`.

Looking up the manual for the ZNB40 VNA in particular, we find a section on independently defining the port configuration via the K4 option ([see this FAQ from R&S](https://www.rohde-schwarz.com/us/faq/reverse-sweep-backward-sweep-faq_78704-39503.html)). Interestingly enough, atleast on the VNA we have at hand I was unable to find this option via the in-built local GUI of the VNA.

It would be nice if the driver could include this feature.

Start with the ZNB driver entry point in qcodes.instrument_drivers.rohde_schwarz.ZNB and read the linked Rohde & Schwarz reverse-sweep FAQ. Inspect how frequency_start and frequency_stop are validated for measurement channels; done means the driver can represent a reverse sweep for a channel without rejecting a lower stop frequency.

Written by the indexing model from the issue text.