Implements comprehensive Akkar et al. (2014) GMPE set

Implements the full set of GMPEs defined by Akkar et al. (2014) including coefficients for Joyner-Boore distance, epicentral distance and hypocentral distance.

AkkarEtAl2013 GMPE is now implemented as AkkarEtAlRjb2014. However, to retain compatibility with large scale regional models using AkkarEtAl2013 we keep the original class active, albeit it now inherits AkkarEtAlRjb2014 with an added deprecation warning. The test tables for AkkarEtAl2013 are also used in AkkarEtalRjb2014 to ensure correctness.