Add measurement column: flux percentage of total island flux

[ajstewart]

A useful metric to filter on as found in previous analysis.

[marxide]

From our earlier discussion for posterity:

We could add a column to Measurement for the fractional source flux (one for peak and one for integrated), then store the aggregated minimum in Source for queries. I suggested storing the maximum earlier, but I think we want the minimum.

Imagine a point source that has an artefact in one epoch such that the fractional source flux for that epoch is 0.98 and 0.02. All other epochs that do not contain an artefact would have single measurements with a fractional source flux of 1.0. We'd want to include this source for consideration as a potential variable if it exceeded the variability metric thresholds and if the minimum fractional source flux was e.g. > 0.97.