added docstrings throughout

psrdada_cpp/cli_utils.hpp

psrdada_cpp/dada_client_base.hpp

psrdada_cpp/dada_dbnull.hpp

psrdada_cpp/dada_io_loop.hpp

psrdada_cpp/dada_io_loop_reader.hpp

psrdada_cpp/dada_io_loop_writer.hpp

psrdada_cpp/dada_junkdb.hpp

psrdada_cpp/dada_read_client.hpp

psrdada_cpp/dada_write_client.hpp

psrdada_cpp/examples/dbnull.cpp

psrdada_cpp/examples/junkdb.cpp

psrdada_cpp/multilog.hpp

psrdada_cpp/raw_bytes.hpp