Fix: Add missing __init__.py to fix installation and tests on Python 3

[AviJxn]

Problem

The package was failing to install correctly via pip install . on environments where the configure script is not run (e.g., standard Python/pip workflows or Windows). This caused csa to be treated as an empty namespace package, leading to NameError failures in the test suite because symbols like cross and full were not exported.

Solution

Manually added csa/__init__.py (based on csa/__init__.py.in) to ensure the package initializes correctly and exports all necessary modules.

Verification

Ran the test suite locally on Python 3.11:
Before Fix:
FAILED (errors=9) - All tests crashed with NameError.

After Fix:
Ran 9 tests in 6.702s
OK