* introduce propagation of C++ exceptions to Cython/Python
* convert all cout/cerr output to exceptions
* Cython: introduce separate 'do_conversion' => avoid constructor to
trigger conversion
- raweat.hpp: find_markers(): properly (re)adjust valid_ flag when doing multiple conversion
- rawmerge.hpp: improve logging, merge_channels(): add condition to avoid range error in vector
- example.py: obtain return flag from add_channel(), write csv output from parquet table
- main.cpp: return to pure raw_eater test version
- implement full app example in pyt/example.py
- merges time series of all consistent channels
- extract them into pyarrow table
- save pyarrow table as compressed parquet file
