Initial implementation of Megawati & Pan (2010) GMPE

Implements GMPE of Megawati & Pan (2010) for large, long distance subduction earthquakes. The GMPE is needed for PSHA in Singapore and Malaysia. 

Note the GMPE has, unsuprisingly, very small acceleration values. Therefore there exist some irreducible discrepancies between this implementation and the test implementation due to rounding of coefficients. Differences appear at about the 8th or 9th decimal place. To allow for this, the maximum discrepancy percentage is raised from the usual 0.1 % to 1 %.