Guard to_lab_frame/to_cm_frame against undefined product/residual

[Copilot]

Reaction.to_lab_frame() and to_cm_frame() unconditionally dereferenced self.product and self.residual, causing an opaque AttributeError for reaction types where these are None (e.g. tot, non, x).

Changes

• reaction.py: Both methods now validate self.product and self.residual upfront, raising a descriptive ValueError before any dereference occurs.

# Before: silent AttributeError at runtime
rxn = Reaction("tot", ...)
rxn.to_lab_frame(theta, Ecm, Q)  # AttributeError: 'NoneType' has no attribute 'm0'

# After: clear ValueError with context
# ValueError: to_lab_frame() requires both product and residual to be defined.
# This reaction (tot) does not have a defined product and/or residual.

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.