OBE printing, post-process h5 I/O, and the partial-projector rename

[harrisonlabollita]

Summary

Three small independent improvements to the obe / post-processing interface: a
terminology rename that brings the code in line with the documentation, operator<< and
HDF5 I/O for structs that were missing them, and one more c++ function exposed to
Python.

This provides an incremental diff:
pr/checkpoint-summarize...pr/obe-printing-h5

Changes

1. theta projector → partial projector (da5248b)

The documentation call it a partial projector, while the code called it theta. To be consistent between the two, I've renamed throughout loaders.{hpp,cpp} and postprocess.{hpp,cpp}. This should improve readability and clarity.

Public Python API change:
one_body_elements_with_theta_projectors → one_body_elements_with_partial_projectors
(python/triqs_modest/obe.toml). Needs a ChangeLog.md entry.

Additional changes in this PR:

• h5_read / h5_write for spectral_function_kw and spectral_function_w
  (h5.cpp) — these results were previously not serializable.
• operator<< for the post-process structs (printing.cpp).
• Rename propagated through load_data_test.cpp and mpi_test.cpp.

2. operator<< for the remaining OBE structs (f318fba)

Adds printing for one_body_elements_tb and one_body_elements_gw, so every OBE
variant is now printable. printing.cpp picks up an obe_tb.hpp include.

3. Expose rotate_local_basis (05af10e)

Added to the obe c2py spec so the local-basis rotation that the post-processing
routines apply internally is reachable from Python. Documented in
doc/reference/python/one_body_elements.rst.

Testing

Existing tests only; load_data_test, mpi_test and obe_tb_test are updated
for the rename and pass.