added compose_emcoinc functionality

Contribution towards emfollow/ligo-followup-advocate!24 (closed)

I have added a "compose_emcoinc" function to "__init__.py" and included a test function in "test_tools.py" to replace "test.py".

I'm not sure how the parameters are supposed to be read in, so I've just created an "em_coinc.json" file in the S1234 files folder.