Modified Campbell2003adjusted and ToroEtAl2002adjusted to pick
the nearest vs30 and kappa that is defined.
Added _compute_soil_amplification method to BooreAtkinson2008Prime,
as a wrapper for the linear and non-linear site amplification functions,
and limited computation to sites with VS30 < 1500 m/s.

Modified _compute_soil_amplification method of AtkinsonBoore2006Prime,
to use interpolation between B/C and HR for sites with VS30 between
760 and 2000 m/s, instead of computing soil amplifications.

Replaced inefficient conversions (base 10 to base e and cm/s2 to g)
with more efficient ones in AtkinsonBoore2008Prime.
Merge branch 'rob-hazardlib' of https://github.com/ROB-Seismology/oq-hazardlib into rob-hazardlib