Thermal channels NinJoTIFF writing fail with AttributeError due to lost attributes

[gerritholl]

Describe the bug

Writing to NinJoTIFF a thermal channel with Celsius units fails with an AttributeError because the dataset attributes are lost in the new unit conversion introduced in #1615

To Reproduce

from satpy import Scene
from satpy.utils import debug_on; debug_on()
sc = Scene(
        filenames=["/media/nas/x21308/scratch/hrpt_metop03_20210323_1809_12327.l1b"],
        reader=["avhrr_l1b_aapp"])
sc.load(["4"])
ls = sc.resample("eurol")
ls.save_dataset("4",
        "/tmp/test.tif",
        writer="ninjotiff",
        physic_unit="C",
        sat_id=7900014,
        chan_id=900015,
        data_cat="PORN",
        nbits=8,
        data_source="HRPT",
        ch_min_measurement_unit=40,
        ch_max_measurement_unit=-87.5)

Expected behavior

Writing a file.

Actual results

bla bla bla, then

Traceback (most recent call last):
  File "/home/gholl/checkouts/protocode/mwe/ninjotiff-crash.py", line 8, in <module>
    ls.save_dataset("4",
  File "/home/gholl/checkouts/satpy/satpy/scene.py", line 993, in save_dataset
    return writer.save_dataset(self[dataset_id],
  File "/home/gholl/checkouts/satpy/satpy/writers/ninjotiff.py", line 202, in save_dataset
    return super(NinjoTIFFWriter, self).save_dataset(
  File "/home/gholl/checkouts/satpy/satpy/writers/__init__.py", line 811, in save_dataset
    return self.save_image(img, filename=filename, compute=compute, fill_value=fill_value, **kwargs)
  File "/home/gholl/checkouts/satpy/satpy/writers/ninjotiff.py", line 174, in save_image
    return nt.save(img, filename, data_is_scaled_01=True, compute=compute, **kwargs)
  File "/data/gholl/miniconda3/envs/py39/lib/python3.9/site-packages/pyninjotiff/ninjotiff.py", line 541, in save
    area_def = img.data.area
  File "/data/gholl/miniconda3/envs/py39/lib/python3.9/site-packages/xarray/core/common.py", line 239, in __getattr__
    raise AttributeError(
AttributeError: 'DataArray' object has no attribute 'area'

Environment Info:

• Satpy Version: master

Additional context

Mea culpa, mea culpa, mea massima culpa. PR coming shortly.