Fixed broken import in typecaster.h (#1181)

- Fixed the broken import _slsdet --> slsdet._slsdet caused by a previous upgrade
- Added tests that exercises the conversion from python to C++ and from C++ to python
- Python unit tests now run in CI (!)

python/slsdet/__init__.py

@@ -41,3 +41,6 @@ def read_version():
     
 __version__ = read_version()
 
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