Update atkinson_2015.py
Update atkinson_2015.py

Changed CoeffsTable, which was completely wrong, to the actual values in the paper.

Former-commit-id: 9fa023050d3fe1cdd634cd2aed10493c485298a9
gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine,gem/oq-engine