`get_box_time` time range minimum

get_box_time only returns sources where the specific ephemera points lie within the requested time range. Since the ephemera are only sampled every hour, this means short time requests will return no sources even if a source is really in the box, since the time range simply didn't contain any ephemera points. One solution to this is to interpolate the ephemera, but that's time consuming, both because we have to make the interpolation object and because it means we can no longer use an SQL query to find objects in the box. The simpler and probably better solution is to put a minimum on the length of time of the time box.