Implement energy-magnitude scale

[gthompson]

Compute the energy-magnitude scale.
To do this, compute waveform energy on each station (Z component or 3-component vector?), correct for distance and Q, to get an energy magnitude for each station. The network magnitude should then be the median of these (or a weighted mean, e.g. weight estimates by distance from median).

Next stage would be to correlate against magnitudes computed by MVO, SRC, Guadeloupe etc.

Could also compute duration magnitudes.