NIST Quert wavelength limits not implemented property

The upper and lower wavelength limits are not being set correctly.

in core.py, _args_to_payload, lines 94 and 95, the min and max wavelengths are set incorrectly. The dictionary key is currently set as
```
request_payload["low_wl"] = min_wav
request_payload["upp_wl"] = max_wav
```
They keys should instead be "low_w" and "upp_w":
```
request_payload["low_w"] = min_wav
request_payload["upp_w"] = max_wav
```

Open core.py and inspect _args_to_payload around lines 94-95, where the NIST query payload assigns the wavelength limits. Update the two payload keys to the names specified in the issue, then verify that lower and upper wavelength limits are sent correctly.

Written by the indexing model from the issue text.