Tweak the 'scientific' template tag to show sub-mJy level fluxes as non-zero.

Since we only display to 3 s.f., sub-mJy fluxes (e.g. 0.0005) were displaying
as 0.000. I've also added an extra digit of precision to the float case, to ensure
that we get at least 2.s.f. of information on every displayed value.
Use 'scientific' format tag for fluxes on image detail page.
Merge pull request #12 from timstaley/units-display-tweak

Tweak the 'scientific' template tag to show sub-mJy level fluxes as non-...