Merge Duplicate Node Indices

[philipc2]

Closes #865

Overview

• Adds the merge_duplicate_node_indices() function that replaces the index of nodes that have the exact latitude and longitude to only reference one node.

Expected Usage

import uxarray as ux

grid_path = "/path/to/grid.nc"

uxgrid = ux.open_grid(grid_path)

# inplace
uxgrid.merge_duplicate_node_indices()

# return a new grid
new_grid = uxgrid.merge_duplicate_node_indices(inplace=False)