Fix `ot` handling in lattice `+` composition

[iamprathosh]

Fixes #146

Implement new ot handling in lattice + composition.

• Add addition lookup table to ot_base in lib/gpt/core/object_type/base.py.
• Implement automatic embedding for complex to ot_singlet lattice in lib/gpt/core/object_type/base.py.
• Add explicit casting functions for explicit embedding/projection in lib/gpt/core/object_type/base.py.
• Add unit tests for new ot handling in lattice + composition in tests/core/expr.py.