Add _replace and others needed to support "copy"

[rajeeja]

gg= ux.open_dataset("test/meshfiles/ugrid/outCSne30/outCSne30.ug", "test/meshfiles/ugrid/outCSne30/outCSne30_var2.nc")
cp_obj = gg.copy()
print(cp_obj)

resulting error:

  File "/Users/mbook/uxarray/uxarray/core/dataset.py", line 51, in _construct_dataarray
    return UxDataArray(xarr, uxgrid=self.uxgrid)
  File "/opt/homebrew/anaconda3/envs/tensorflow/lib/python3.10/site-packages/xarray/core/common.py", line 268, in __getattr__
    raise AttributeError(
AttributeError: 'UxDataset' object has no attribute 'uxgrid'

It'd be good to know what others we might need, this discussion is relevant:pydata/xarray#3980

[erogluorhan]

This has been adressed