Add iAssembly class for creating UC2 cube assemblies in 50x50x55mm grids

[Copilot]

This PR adds comprehensive support for creating complex UC2 assemblies in grid patterns, addressing the need to place UC2 cubes based on a table with specific positions and orientations.

New Features

iAssembly Class

• New iAssembly class for working with Inventor assembly documents (.iam files)
• Grid-based component placement with configurable spacing (default 50x50x55mm)
• Support for rotations around X, Y, and Z axes
• Batch component placement from table definitions

Key Methods

# Create assembly with UC2 grid spacing
assembly = iAssembly('UC2_Assembly.iam', units='metric')
assembly.set_grid_spacing(50.0, 50.0, 55.0)

# Place components as specified in the issue
assembly.place_component_at_grid('Assembly_cube_lens.iam', 0, 0, 0, rotation=(0, 0, 0))
assembly.place_component_at_grid('Assembly_cube_mirror.iam', 1, 0, 0, rotation=(0, 90, 0))

Utility Functions

• CSV-based component definition and import
• Rectangular grid generation
• Alternating/checkerboard patterns
• Component validation helpers

Example Usage

The implementation directly addresses the issue example:

# Assembly_cube_lens.iam at 0,0,0 at orientation 0,0
# Assembly_cube_mirror.iam at 50,0,0 at angle 0,90

from PyInventor import iAssembly

assembly = iAssembly('UC2_Grid.iam', units='metric')
assembly.set_grid_spacing(50.0, 50.0, 55.0)

assembly.place_component_at_grid('Assembly_cube_lens.iam', 0, 0, 0, (0, 0, 0))
assembly.place_component_at_grid('Assembly_cube_mirror.iam', 1, 0, 0, (0, 90, 0))
assembly.save()

Files Added

• Core Implementation: Updated PyInventor/pyinvent.py with iAssembly class
• Examples: uc2_grid_example.py, simple_uc2_issue_example.py
• Utilities: uc2_grid_utilities.py with helper functions
• Sample Data: sample_uc2_components.csv for CSV workflow
• Documentation: UC2_Grid_Documentation.md with complete API reference
• Tests: test_uc2_grid.py for validation

Grid Coordinate System

• Grid coordinates are integers (0, 1, 2, ...)
• Actual positions = grid_coords × grid_spacing
• Example: Grid (1,0,0) with 50mm spacing → Position (50,0,0)mm

CSV Workflow Support

name,file_path,grid_x,grid_y,grid_z,rot_x,rot_y,rot_z
Lens_Origin,Assembly_cube_lens.iam,0,0,0,0,0,0
Mirror_50mm,Assembly_cube_mirror.iam,1,0,0,0,90,0

Backward Compatibility

• All existing PyInventor functionality preserved
• New features are additive and don't affect existing iPart class
• Updated __init__.py to export iAssembly alongside existing classes

Fixes #3.

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