Function to fill NaN with a value (e.g. 0.0)

[johnkouman]

Is your feature request related to a problem? Please describe.
I had a dataset with some items inside and some items had NaN. In pandas and xarray there is the function [.fillna(value)] which fills the NaN with the value. I searched in mikeio lib and didn't find it or a similar.

Describe the solution you'd like
To solve the issue and continue, I copied the dataset, I converted to xarray, applied the .fillnan and replaced the 'new' values to dataset.

dfsu_concat = dfsu_concat.copy()
    dfsu_concat_xray = dfsu_concat.to_xarray().fillna(0)
    variable_values = {
        var_name: dfsu_concat_xray[var_name].values
        for var_name in dfsu_concat_xray.data_vars
    }

dfsu_concat = mikeio.Dataset(
      data=variable_values.values(),
      time=dfsu_concat.time,
      items=dfsu_concat.items,
      geometry=dfsu_concat.geometry,
  )