hazard location (site) behaviour

[chrisdicaprio]

We, as users of runzi, want to be able to set site-specific vs30 values. This is done by specifying locations in a csv file (rather than using the locations variable in the config toml) each with it's own vs30.

The file is specified by locations_file and cannot be set if locations is set (and vice-versa)

Possible behaviours:

1. if uniform vs30 specified, ignore for the site specific values
2. if uniform vs30 specified, raise exception
3. require uniform vs30 = 0

We will implement option 2 as it is least error prone / subject to misinterpretation.