Fix clip_domain broadcasting error for extents strictly inside the domain

[martinguthrie93]

Summary

Fixes #557.

pysteps.utils.dimension.clip_domain raised

ValueError: could not broadcast input array from shape (a,b) into shape (c,d)

whenever the requested clipping extent lay strictly inside the source domain. Since clip_domain is the standard way to crop a precipitation field to a geographic bounding box, any workflow that clips to an interior region of interest hit this.

Root cause

The source and destination copy windows were computed independently, using strict pixel-center comparisons (x_coord < right_ / > left_) against two different reference boxes (the requested extent vs. the full source domain). When the extent was strictly interior, the strict comparison dropped a pixel on one side and the two windows ended up with different sizes, so the final assignment failed to broadcast.

The previous tests only used extents whose edge coincided with the source-domain boundary, a special case where the window sizes happened to match, so the bug went unnoticed.

Fix

Because clip_domain preserves the pixel size, clipping is an integer-pixel window operation. The window is now recomputed from a single pixel offset between the clipped grid and the source array (col_offset / row_offset, the latter accounting for the yorigin convention), so the source and destination windows always match in size.

Additional benefits of the offset-based formulation:

• extents that extend beyond the source domain are now padded with metadata["zerovalue"] instead of raising;
• the returned metadata (x1, x2, y1, y2) stays consistent with the clipped pixel grid and preserves the pixel size.

For the fully-contained case used elsewhere in the test suite (e.g. the 512x512 MCH clip in helpers.py), the returned array is byte-for-byte identical to the previous behaviour.

Tests

pysteps/tests/test_utils_dimension.py:

• interior clip that previously raised, for both yorigin="upper" and yorigin="lower";
• an out-of-domain extent that must be zero-padded;
• a check that the returned metadata describes the clipped pixel grid.

All existing clip_domain cases still pass, and black reports no changes.