Add stream-based loading to Python bindings

[samusangio]

Add stream-based file loading to the Python bindings so callers can parse spectra without writing to disk.

• Add SpecFile.loadFileFromStream(stream, pseudo_filename, file_ending_hint="") with Auto parser ordering hints (bindings/python/SpecFile_py.cpp).
• Document usage and add coverage for file objects, io.BytesIO, and a clear error for non-seekable streams (bindings/python/README.md, bindings/python/tests/test_specutils.py).
• Avoid leaving Python test artifacts by using a temp dir + ignoring common Python/test outputs (bindings/python/tests/test_specutils.py, .gitignore).

with open("spectrum_file.n42", "rb") as f:
    spec.loadFileFromStream(f, "spectrum_file.n42")