Fix off-center, defocused SCAPE stage-scan acquisitions (#404)

[bls337]

Fix off-center, defocused SCAPE stage-scan acquisitions (#404)

During a stage scan the light sheet and imaging piezo were positioned at the piezo home position (0) instead of the imaging center, so the acquired volume was off-center and out of focus. This restores the single-objective (SCAPE) behavior from the diSPIM 1.4 plugin:

• Place the galvo/sheet at the imaging center (not piezo home) so a stage scan images the same plane as a galvo scan.
• Move the imaging piezo to the imaging center instead of homing it to 0 (keeps the scan in focus).
• Leave the piezo at the imaging center after acquisition instead of parking it at 0.

Verified on hardware against the diSPIM 1.4 plugin on the same controller.

Note: a separate scan-direction issue is still present after this fix