Default 'ivert validate -ex' bboxes to W/E/S/N order with --wsen flag

[mmacferrin]

Summary

The `-ex`/`--exclude` bounding boxes in `ivert validate` were parsed in W/S/E/N (`minx/miny/maxx/maxy`) order, inconsistent with `ivert database` commands, which default to W/E/S/N (`minx/maxx/miny/maxy`) with an opt-in `--wsen` flag.

This PR makes `ivert validate` behave the same way:

• `-ex`/`--exclude` bounding boxes now default to W/E/S/N (`minx/maxx/miny/maxy`).
• A new `--wsen` flag switches interpretation to W/S/E/N (`minx/miny/maxx/maxy`), mirroring `ivert database download`.
• Downstream `validate_dem` still receives `(minx, miny, maxx, maxy)` tuples, so nothing below the CLI layer changes.

Changes

• `_parse_exclude_spec(value, wsen=False)` reorders the four values based on the flag and returns the same tuple shape as before.
• Updated `-ex`/`--exclude` help text to document the new default order and the `--wsen` switch.
• Added the `--wsen` flag to the `validate` command and threaded it through.

Verification

• `_parse_exclude_spec('1/2/3/4')` → `(1.0, 3.0, 2.0, 4.0)` (default W/E/S/N)
• `_parse_exclude_spec('1/2/3/4', wsen=True)` → `(1.0, 2.0, 3.0, 4.0)` (W/S/E/N)
• `ivert validate --help` shows the updated `-ex` help and the new `--wsen` option.