Optimize FlatStruct and applyFlat for memory efficiency

[Cybis320]

Optimize FlatStruct and applyFlat for memory efficiency and bit depth flexibility

Changes

• Refactored FlatStruct to dynamically handle both 8-bit and 16-bit flat field images
• Updated applyFlat function to work with the new FlatStruct
• Implemented memory-efficient storage using appropriate integer types (uint8 or uint16)
• Eliminated redundant float64 arrays
• Implemented in-place calculations to minimize memory allocation

Motivation

Addresses #392

[Cybis320]

FlstStruct went from 33MB to 4MB for a 1080p flat.