Add seismic tag/plots for ground motion exceeding threshold

This is the counterpart to the wind followup submitted recently. This followup checks the z-axis ground motion around the time of a lockloss against a site-specific threshold and creates the 'SEISMIC' tag if it exceeds said threshold. Regardless of whether or not the threshold is exceeded, a plot of the BLRMS velocity and the raw velocity sensor output ('seismic.png') is created and displayed in the 'Environment Plots' section.

Example EQ lockloss at LHO: https://ldas-jobs.ligo-wa.caltech.edu/~yannick.lecoeuche/index.cgi?event=1249101030

Example EQ lockloss at LLO (event directory): https://ldas-jobs.ligo-la.caltech.edu/~yannick.lecoeuche/events/1250/005567/

Also includes a small change to the windy followup which lets it obtain the maximum windspeed between the three channels even if those channels do not exceed the wind threshold for the site.