Fix unclosed file warnings

[lucduron]

No description provided.

[lucduron]

Here is a script to display warnings:

import xarray as xr

from xarray_selafin.xarray_backend import SelafinBackendEntrypoint

import warnings
warnings.simplefilter("default")

slf_in = "tests/data/r3d_tidal_flats.slf"
ds = xr.open_dataset(slf_in, engine=SelafinBackendEntrypoint)
ds.selafin.write("OUT.slf")
ds.close()

The generated output:

ResourceWarning: unclosed file <_io.BufferedWriter name='OUT.slf'>
  write_serafin(filepath, ds)
ResourceWarning: Enable tracemalloc to get the object allocation traceback
sys:1: ResourceWarning: unclosed file <_io.BufferedReader name='tests/data/r3d_tidal_flats.slf'>

It shows that without this PR, there are 2 warnings:

• on the opened file to read
• on the opened file to write

[tomsail]

this one looks good to me.
@pmav99 do you have any comments on the tests?