[Solver/Pieces] Voxel coloring

[mbrown1413]

Allow for voxels to be colored when defining pieces. When solving, a voxel's color must match the goal's color at that position to be considered a valid placement.

This issue is a subset of #57, which is for more generalized voxel and side properties that can be used in arbitrary ways by extensions.

• Ability to color voxels
• Ability to map piece colors to a list of goal colors which they are allowed in
• Implement solver code to limit placements
• Importing colors from BurrTools .xmpuzzle files