`Brain_Data` fixes & enhancements

[ejolly]

Relevant issues:

• Expand/collapse mask sometimes creates extra mask values #126
• Add tests to handle custom masks passed to Brain_Data #150
• Add ignore_attrs flag to Brain_Data append #391
• Slice time correction defaults #420
• Clean up regress #434

[ejolly]

v0.6.0 Progress Update

Significant work has been completed for this release:

• ✅ Brain_Data constructor flattens multiple files instead of preserving subject structure #449 fixed: Brain_Data constructor now correctly preserves multi-subject structure when loading file lists
• ✅ Mask handling improved: Better mask validation and error messages
• ✅ Constructor refactored: Cleaner initialization logic with better type handling
• ✅ HDF5 I/O updated: Improved save/load with better attribute preservation

Still open:

• Expand/collapse mask sometimes creates extra mask values #126 - Expand/collapse mask edge cases
• Slice time correction defaults #420 - Slice time correction defaults
• Clean up regress #434 - regress() enhancements (VIF check, scaling options)