added extract_decays.py

added script to extract the decays for a provided particle from a
Decay.Dec file (path provided by the user)
extract_decays is now a function and handles erros

extract_decays is now a function, so it can be imported
it has error handling if the file does not exist, the Decay is not
exactly found (no whitespace after decay, should this change?) or
Enddecay is not found
initial add of tests for extract_decays.py

some first tests for extract_decays.py