Add comprehensive unit tests for colvartypes and colvarvalue

[Copilot]

Summary

Adds 3 new unit test files to improve test coverage of core mathematical and data types in the Colvars library.

Changes

New test files

tests/unittests/colvartypes_vectors.cpp
Tests for cvm::rvector (construction, arithmetic, dot/cross product, norm, unit vector) and cvm::vector1d (construction, arithmetic, inner product, norm, sum, slice, serialization round-trip).

tests/unittests/colvartypes_quaternion.cpp
Tests for cvm::quaternion (construction, arithmetic, Hamilton product, conjugate, norm, set_positive, rotation of 3-vectors, rotation matrix, dist2, dist2_grad, cosine, as_vector/get_vector conversions).

tests/unittests/colvarvalue_all_types.cpp
Tests for colvarvalue across all supported types — scalar, 3-vector, quaternion, generic vector:

• Construction from type flag, base type, and copy constructor
• Binary arithmetic (+, -, *, /)
• In-place operators (+=, -=, *=, /=)
• Inner product
• norm2(), norm(), sum()
• dist2(), dist2_grad()
• interpolate()
• apply_constraints()
• set_ones(), reset()
• is_derivative()
• Type metadata: type_desc(), type_keyword(), num_dimensions(), size()

tests/unittests/CMakeLists.txt — Adds a new foreach loop to build and register the 3 new tests (without requiring the colvars_stubs proxy library, since these tests only exercise core mathematical operations).

Testing

All 8 unit tests pass (5 original + 3 new):

• colvarvalue_unit3vector ✅
• file_io ✅
• memory_stream ✅
• read_xyz_traj ✅
• parse_error ✅
• colvartypes_vectors ✅ (new)
• colvartypes_quaternion ✅ (new)
• colvarvalue_all_types ✅ (new)