Rethink python send functionality

[tobyjamez]

The functionality to send data within the python API is not an intended usecase. It is useful as a utility for testing the receiving APIs. Further work would be required to make the python send functionality consistent with the python receive functionality and the C++ send functionality, with only marginal benefits. It is probably best to reduce the size of the python send functionality and make it clear that this not intended as part of the user-facing API, rather simply as a utility within the codebase.