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)

[aliFrancis]

Hey! Could you revert the commits here and just commit the code changes, leaving all the auto formatting from ruff out?

Sorry, I know it's a pain to do but I don't want to make so many line changes with this commit as it makes it hard to see what is the important part you've fixed.

[aliFrancis]

Covered by #10

Closing now. Thanks!