Add pathlib support in `save_dataset` (and elsewhere)

Feature Request

Is your feature request related to a problem? Please describe.

Scene.save_dataset accepts a path only as a string. It does not accept pathlib.Path objects.

Describe the solution you'd like

I would like that Scene.save_dataset accepts pathlib.Path objects.

Describe any changes to existing user workflow

There is no reasonable way in which adding this can break anybody's workflow.

Additional context

I'm not sure if this belong here or in trollsift. When attempting to write to a pathlib.Path object, the exception is actually coming from trollsift:

Traceback (most recent call last):
  File "mwe30.py", line 7, in <module>
    sc.save_dataset("tofu", Path("/tmp/tofu.png"))
  File "/data/gholl/miniconda3/envs/py38/lib/python3.8/site-packages/satpy/scene.py", line 1301, in save_dataset
    return writer.save_dataset(self[dataset_id],
  File "/data/gholl/miniconda3/envs/py38/lib/python3.8/site-packages/satpy/writers/__init__.py", line 829, in save_dataset
    return self.save_image(img, filename=filename, compute=compute, fill_value=fill_value, **kwargs)
  File "/data/gholl/miniconda3/envs/py38/lib/python3.8/site-packages/satpy/writers/simple_image.py", line 62, in save_image
    filename = filename or self.get_filename(**img.data.attrs)
  File "/data/gholl/miniconda3/envs/py38/lib/python3.8/site-packages/satpy/writers/__init__.py", line 658, in get_filename
    output_filename = self.filename_parser.compose(kwargs)
  File "/data/gholl/miniconda3/envs/py38/lib/python3.8/site-packages/trollsift/parser.py", line 54, in compose
    return compose(self.fmt, keyvals)
  File "/data/gholl/miniconda3/envs/py38/lib/python3.8/site-packages/trollsift/parser.py", line 409, in compose
    return formatter.format(fmt, **keyvals)
  File "/data/gholl/miniconda3/envs/py38/lib/python3.8/string.py", line 163, in format
    return self.vformat(format_string, args, kwargs)
  File "/data/gholl/miniconda3/envs/py38/lib/python3.8/string.py", line 167, in vformat
    result, _ = self._vformat(format_string, args, kwargs, used_args, 2)
  File "/data/gholl/miniconda3/envs/py38/lib/python3.8/string.py", line 177, in _vformat
    self.parse(format_string):
  File "/data/gholl/miniconda3/envs/py38/lib/python3.8/string.py", line 261, in parse
    return _string.formatter_parser(format_string)
TypeError: expected str, got PosixPath

There may be other I/O functionality in satpy specifically and pytroll more generally where a path is currently accepted only as a string and not as a pathlib.Path object.

Start at Scene.save_dataset and follow the traceback through satpy/writers/init.py, satpy/writers/simple_image.py, and the trollsift parser. Check the related I/O paths mentioned in the issue and establish coverage showing that pathlib.Path values are accepted wherever string paths are supported.