Fix get_bounded_footprint and remove unused imports

[frandorr]

• Fix error in get_bounded_footprint what was wrongly indexing by row instead of row_idx

Traceback (most recent call last):
  File "/home/fran/repos/Major-TOM/src/grid.py", line 318, in <module>
    bbox = grid.get_bounded_footprint(point, buffer_ratio=0.1)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/fran/repos/Major-TOM/src/grid.py", line 218, in get_bounded_footprint
    max_col = len(self.points_by_row[row].col_idx) - 1
                  ~~~~~~~~~~~~~~~~~~^^^^^
TypeError: list indices must be integers or slices, not numpy.str_

• Remove unused imports from grid
• Remove unused variables in get_utm_zone_from_latlng
• Autoformat the code using Ruff (sorry, this was not intentional)