clip magnitude at 8.3
added methods for calculating distances to volcanic front and subduction trench
implemented methods for subduction trench and volcanic front corrections
added GMPEs for North-East and South-West corrections and updated tests tables for total std (total std is computed with mean not-corrected)
added tests for corrected GMPEs
optimized subduction trench distance calculation
reduced size of verification tables
Merge pull request #227 from monellid/si_midorikawa_corrections

Implement corrections for Si & Midorikawa 1999 GMPE